How to Tell if Your Computer Is Infected by a Trojan Horse

Co-authored by Michael Thompson-Brown and Nicole Levine, MFA

Last Updated: September 22, 2025

A trojan horse is a type of malware that can infect any computer. Trojans find their way onto computers by hiding in software downloads, making them easy to (unintentionally) install. You can also get a trojan horse by visiting an insecure or malicious website. Once a trojan horse is installed on your computer, it can spy on you, steal your personal information, and/or create backdoors that allow other hackers to do the same. This wikiHow teaches you how to tell if your computer is infected with a trojan horse, and how to secure your computer.

Things You Should Know

Be wary if your computer is running especially slowly, programs are unexpectedly opening, or you've noticed an influx of spam and pop ups.^{[1]
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Install and/or update your antivirus software and run a scan. Most computers have built in antivirus software, but your computer must be up to date for them to run effectively.^{[2]
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To run a scan on a Mac, you'll also need to download third party software such as Malwarebytes for Mac.^{[3]
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Steps

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Recognize the signs. The signs of a trojan horse can be similar to other types of viruses and malware. You may have a trojan horse if you are experiencing any of the following:
- Is your computer running more slowly than usual? Trojans run software in the background that can use a lot of precious computing power. A "zombifying" trojan can even make it impossible for you to use your computer at all while the hacker is using it to attack a network.^{[4]
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- Are you seeing programs that you don't recognize, or are programs opening automatically that weren't before? Trojans install software on your computer that can be used to harvest your information or use your computer to attack others. You may not see these programs in your Start menu or Apps folder, but sometimes you'll see them running in the background.
- Have you noticed a lot of pop-up windows or spam? Trojans may install programs that cause pop-up windows to appear on the screen, sometimes asking for login or banking information. If you see a pop-up asking you for personal information, don't enter that information unless you specifically visited the website or opened the app first.
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Install and/or update your antivirus software. Both Windows and macOS come with built-in security software designed to keep you safe from trojan horses and other threats. However, if your computer and antivirus software are not up to date, newer trojans may creep through.
- If you're using Windows 10, Windows Security is always running in the background, doing frequent scans for trojan horses and other threats.^{[6]
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  Go to source} To make sure it's up to date, press Win + i to open your Settings, click Update & security, and then click Check for updates. Install any updates that are found.
- If you have a Mac, virus and malware protection is built in to your computer already.^{[7]
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  Research source} Apple recommends keeping your system up to date to ensure the best protection. Click the Apple menu, select System Preferences, click Software Update, and then click Update Now if any updates are found.
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Scan your computer. Any reputable antivirus/antimalware software can check your computer thoroughly for trojan horses. If your software detects a trojan horse, it will let you know, and then help you remove it from your computer.
- Windows Security is great at tracking down and removing most threats on its own, but for a deeper scan, you can run an offline scan. In the Windows search bar, type virus, click Virus & threat protection in the search results, and then click Scan Options. Select Microsoft Defender Offline Scan and click Scan Now.
- Although you have antivirus protection, you can't run a scan on a Mac without installing non-Apple antivirus/antimalware software. One of the most well-known available is Malwarebytes for Mac, and you can use it to scan your Mac for free. Download Malwarebytes from https://www.malwarebytes.com/mac-download, and then double-click the downloaded file to install the software. Once installed, open Malwarebytes and click Scan Now to start the scan.
- There are many other antimalware programs you can use on both Windows and macOS. Malwarebytes, Avast, and AVG are all available for both operating systems, and they all have free scanning options. Each of these programs also offers you the option to pay for an upgrade that will always run in the background so it can catch trojan horses and other malware in real-time.
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Protect yourself from trojan horses in the future. Whether your antivirus scanner found a trojan horse or not, you'll still want to do everything you can to make sure you stay protected from threats in the future.
- Keep all of your software up to date. When Windows or macOS prompts you to update to the latest version of the operating system, do it as soon as possible. While these update notices don't always pop up at the most convenient times, they are timely—updates usually contain security updates that fix issues that hackers can exploit. The longer you wait, the more vulnerable you are to attack.
- Never install applications from sources you don't know and trust. Although it's not fail-proof, you'll have a better chance of avoiding trojan horses if you stick to installing software through the App Store app on your Mac or the Microsoft Store app on your PC.
- Never open attachments in email messages unless you're expecting the attachment. Trojan horses may infect your computer through a program file sent by someone else—even if the email message is from someone you trust, there's a chance a virus infected their computer and is now trying to install trojan horses on other peoples' computers.
- Avoid visiting risky websites. If your browser warns you that your connection is not secure, or you're being inundated with pop-ups or fake virus notices (these can look convincing!), close your browser tab, open a new one, and browse to a different website.

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What's the best anti-virus program?

Michael Thompson-Brown
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Michael Thompson-Brown is a Cybersecurity Expert based in Portland, Maine. He is the owner and founder of PCRescue!, a company that provides cybersecurity and data management solutions to small businesses and individuals. Michael has over 25 years of experience in the IT industry and is a certified ethical hacker. He is passionate about helping clients protect their data and systems from malicious attacks and helping them optimize their online presence and reach. He received a Master’s degree in Cybersecurity and Information Assurance from Western Governors University, and a Bachelor’s degree in Business Administration from the University of Phoenix.

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The best antivirus program is often the built-in one, as it receives daily updates and runs smoothly in the background without consuming excessive resources. These built-in solutions seamlessly integrate with the operating system, ensuring efficient utilization of resources and optimal performance.

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How do I know if my computer has malware?

Michael Thompson-Brown
Cybersecurity Expert
Michael Thompson-Brown is a Cybersecurity Expert based in Portland, Maine. He is the owner and founder of PCRescue!, a company that provides cybersecurity and data management solutions to small businesses and individuals. Michael has over 25 years of experience in the IT industry and is a certified ethical hacker. He is passionate about helping clients protect their data and systems from malicious attacks and helping them optimize their online presence and reach. He received a Master’s degree in Cybersecurity and Information Assurance from Western Governors University, and a Bachelor’s degree in Business Administration from the University of Phoenix.

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To determine if your computer has malware, open Task Manager by right-clicking on the taskbar and selecting "Task Manager." In Task Manager, navigate to the "Processes" tab and scan through the list for any unfamiliar or suspicious processes and applications. Look for processes with random or nonsensical names, high CPU or memory usage without any apparent reason, or processes running from unfamiliar locations on your system. These could indicate the presence of malware, and further investigation or removal may be necessary.

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I got a notification that I got a Zeus virus and I called and they said that I have a trojan and that it costs 100 dollars to fix. Is this a scam?

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Yes, this is a scam.

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Tips

If a pop-up advertisement prompts you to download anti-virus software, do not install it. This is a common trojan horse tactic. Stick to known products like those mentioned in this wikiHow.

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If you're using an antivirus program that doesn't do regular scans in the background automatically, make sure to turn the "automatic scanning" feature (or similar) on.

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