Rootkits are becoming an increasingly dangerous problem for networked computers, they're constantly becoming more sophisticated, and getting better at evading detection software. This means that your detection methods and tools have to become more effective in order to find and remove this software.
Sysinternals is offering a freeware tool called RootkitRevealer, that can detect some rootkits. Used in conjunction with good anti-virus and malware detection tool you should be able to detect most rootkits if they have invaded your system.
If you don't know what a rootkit is, here is a definition: A rootkit is a collection of tools used by a hacker or a malicious program to try to gain administrator access to your computer. If a hacker is using a rootkit to take advantage of your system, he will generally do this to steal data or cause damage. If a malicious program is trying to break into your system, it will generally open a backdoor for a hacker to take over your system or monitor your keystrokes so it can steal your passwords. Rootkits will also generally modify your computer system files to avoid detection.