One of the most feared of all types of malware is the rootkit, software that is nearly undetectable--and in fact, can't be detected by many, if not most, anti-malware--and can then be used for all kinds of nefarious purposes.
Not uncommonly, rootkits are used to hide Trojans, that can then be used to take over your PC without your knowledge.
The free AVG Anti-Root is designed to protect you. It does one thing, and one thing alone: finds and kills rootkits. Run it and it scans your PC, sniffing them out. When it finds any, it removes them. Note that it doesn't work with Windows Vista.