Here are some things u can use to keep your computer clean against a bunch of nasty viruses and stuff. (malware)
it might be slightly outdated, cuz i made this post on a different forum a long time ago, but it shouldnt be too bad.
spyware blaster (anti spyware)
SpywareBlaster - Reviews and free SpywareBlaster downloads at Download.com
pctools firewall (obviously a firewall)
PC Tools Firewall Plus Free Edition - Reviews and free PC Tools Firewall Plus Free Edition downloads at Download.com
ad-aware personal scanner (scans for most malware)
Ad-Aware 2007 - Reviews and free Ad-Aware 2007 downloads at Download.com
advanced windows care (all in one system maintainance)
PC Repairs Freeware | Advanced WindowsCare Personal | Free PC Repair Fix Software Service | Computer Cleaner
AVG free anti-virus
AVG Free Advisor - AVG Anti-Virus Free Edition
AVG free anti-spyware
AVG Free Advisor - AVG Anti-Spyware Free Edition
Microsoft Windows Defender
Download details: Windows® Defender
Spybot Search and Destroy (andti spyware and prevention unit, pretty much like condoms for you computer)
The home of Spybot-S&D!
Tarus scan (i havnt acually tried this out so im not sure if it is good)
Tauscan Update
McAfee site advisor (tells which sites may be bad, and which are clean)
Protection from Adware, Spam, Viruses, Online Scams | McAfee SiteAdvisor
Registy fix (speed up your system by removing old crappy regitry entities)
Registry Repair, Clean Up & File Fix for Windows
Xoftspy SE (a really fast scanner)
XoftSpySE
PC Tools Spyware Doctor (free trial version)
Spyware Doctor - Best Spyware Removal. Free Trial Download.
PC Tools Antivirus (free trial version)
PC Tools AntiVirus - Free Anti-Virus Download