First try to use System restore to restore your system to a time when it worked properly
here's how to do it in xp
http://www.bleepingcomputer.com/tutorials/tutorial56.html#restore
here's how to do it in vista
http://www.bleepingcomputer.com/tutorials/tutorial143.html#restore
After that Uninstall whatever Antivirus Program(s) you are using
and Download a 30 day free Trial of Kaspersky Internet Security
http://www.download.com/Kaspersky-Internet-Security/3000-2239_4-10012072.html?tag=lst-2
configure it for highest protection
http://support.kaspersky.com/kav6mp2/scan?qid=197743848
and scan
After that I recommend buying it, it's worth it.
If you can't afford it
These 3 free tools will help you keep your computer clean ~ Patrick
Check out the video review below for the best free Antivirus program
The Top Rated Free Antivirus Program researched from those who test Antivirus programs (av comparatives, virus.gr, and other reviews including this video review) http://www.youtube.com/watch?v=KhXZo3dHulc is
Avira Antivir http://www.download.com/Avira-AntiVir-PersonalEdition-Classic/3000-2239_4-10322935.html?tag=lst-1
Today you are in more danger from Spyware I Download, Install, Update and Run SuperAntiSpyware
http://www.download.com/SUPERAntiSpyware-Free-Edition/3000-8022_4-10523889.html?tag=lst-2
( I used this to remove spyware and Trojans from a laptop that adaware would only detect but not get rid of ) It detected and removed everything
Download and Run Advanced Windows Care Personal
http://www.download.com/Advanced-WindowsCare-Personal/3000-2086_4-10407614.html?tag=lst-1, this will tune up your system and show you what starts at bootup, and allow you to edit this.
(uncheck the Registry tab, but check the Spyware, Security and startup tabs and see what is starting up and disable harmful items.