All of them work but the truth is that is really hard to find one that covers all the viruses, try to use one that get frequently updated, this is the combination of free softwares that i recommend because i have being using them for over 2 years then when i change to vista they still works great. Other thing you should do is avoid untrusted sites because they are the source of malware, also be careful when downloading from the web scan every file after downloading but before running it.
Download and install the following softwares, then restart your computer in safe mode( if you don't know how click this link http://www.pchell.com/support/safemode.s...
then when windows start scan your computer and heal all the infected files, also try an antispyware.
1-. AVG free edition (antivirus only), get it free at http://www.grisoft.com/doc/download-free...
2-. Ad.Aware 2007 (antispyware)
http://www.download.com/Ad-Aware-2007/30...
3-. Spybot search and destroy (antispyware)
http://www.download.com/Spybot-Search-De...
4-. ccleaner (increase computer performance deleting unused entries and registries.
http://www.download.com/CCleaner/3000-21...
5.- Zone alarm (firewall) Equip your system to deal with Web-based security threats including hackers, viruses, and worms.
http://www.download.com/ZoneAlarm-Securi...
What i recommend is to use the spybot S D only once per week. Then use AVG and Ad-aware as default for daily use. Use ccleaner two times per week to keep your PC running smoothly, Never install more than one antivirus, as they may conflict with each other.On the other hand, is always good to use more than one anti spyware