If you have been installing a lot of free software or warez then it's likely you have malware or spyware of some sort. Its a nightmare

I rooted three keyloggers out of my PC recently which were missed by all the anti virus and spyware software on my PC (which is quite a lot)

Good ones I've found are:
SpyBot (free - picks up various dodginess others miss)
HijackThis (also free. Doesnt work in the same way as others. You need to scan your pc with it and post the results up on a forum where people will root through them and check them. I found two keyloggers using this that all others including Spybot missed)

I also found another nasty keylogger using this online scan:
http://housecall.trendmicro.com/

Finally try using msconfig (type msconfig in Start Menu > Run) go to the Startup tab and do a Google search on each entry in the list. It is possible to find various viruses etc by doing this and you can also clean up your system an awful lot. :)