A keylogger is a piece of hardware or software that records your keystrokes (i.e., what you type into your computer) then either saves them to a log file or transmits them to a third party. Examples of legitimate keyloggers include parental or employer controls. Malicious keyloggers hide on your computer and steal confidential or security-related information such as passwords, bank accounts, or credit card numbers.
Monitor your computer's behavior. If a keylogger displays any symptoms, those symptoms will be similar to other virus behavior: slow computer performance, strange delays, new desktop or system tray icons, excessive hard drive or network activity, etc. If you are experiencing these symptoms, you can determine the nature of the infection by running an anti-virus scan.
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Run an anti-virus scan. If you have an anti-virus program installed, obtain the latest updates and run a full system scan. This should find and clean the majority of keylogger infections, along with the Trojan or worm that may have accompanied it. If you don't have anti-virus protection installed, there are free scanning options available. See the Resources section for links to a few of them.
Verify your process threads. Your computer keeps track of all of its running processes or "threads." A keylogger may have its own thread, or it may be disguised as a system thread. You can verify the legitimacy of your running threads by finding them on a website such as the Process Library (see Resources). To view your running threads, click "Ctrl-Alt-Delete" to access the Task Manager, then click "Processes."
Check your list of installed programs. Legitimate keyloggers can still be used for malicious purposes; if this is the case, the keylogger won't display any unusual side effects, and your anti-virus software won't flag it. Check your list of installed programs for anything unfamiliar or suspicious.
Perform a visual check of your computer's hardware for anything unexpected or unusual. A keylogger can also be a piece of hardware attached to your computer, for instance via a USB or PS/2 port. This is far less common, though, than the software variety.