How to Know If Your Computer Has a Virus

Learn the warning signs of computer viruses and malware. Identify suspicious behavior, performance issues, and security threats on your PC or Mac.

1. Check system performance and speed. Monitor your computer's startup time and general responsiveness. Viruses consume system resources, causing noticeable slowdowns during boot, file opening, and program launches. Open Task Manager (Ctrl+Shift+Esc on Windows) or Activity Monitor (Applications > Utilities on Mac) to identify processes using excessive CPU or memory.
2. Look for unexpected pop-ups and advertisements. Watch for pop-up windows appearing when no browser is open, or ads showing up in unusual locations like your desktop or within non-web applications. Adware and browser hijackers generate these unwanted advertisements to generate revenue. Close these pop-ups using the X button, never by clicking the ad content itself.
3. Monitor network activity and data usage. Check your internet data usage for unexplained spikes that could indicate malware transmitting information. On Windows, open Settings > Network & Internet > Data usage. On Mac, go to System Preferences > Network, select your connection, and click Advanced > TCP/IP. Trojans and spyware often send data to remote servers without user knowledge.
4. Examine browser changes and redirects. Notice if your homepage, default search engine, or new tab page changed without your permission. Browser hijackers modify these settings to redirect traffic through malicious sites. Check for unfamiliar browser extensions or toolbars in your browser's extension management section. Remove any extensions you did not install intentionally.
5. Review file system for suspicious files. Scan your desktop, downloads folder, and temporary directories for files you did not create. Look for executable files with random names, duplicate files with slight name variations, or files with double extensions like document.pdf.exe. Check your recent documents list for files you did not open.
6. Test antivirus software functionality. Verify your antivirus software is running and updating properly. Some viruses disable security software to avoid detection. Open your antivirus program and confirm real-time protection is active and definitions are current. Run a full system scan and note if the software encounters errors or fails to complete.
7. Check system and security logs. Review Windows Event Viewer (type eventvwr in Start menu) or Mac's Console app for security warnings, application crashes, or system errors that coincide with suspicious behavior. Look for failed login attempts, unauthorized software installations, or security software alerts. Document any patterns or timestamps that align with performance issues.