There are a few choices when it comes to security, mainly in the AntiVirus department. Some very good, free anti-viruses available to download are:
1) AVG 8 AntiVirus. Easiest interface to use, very fast scanning and doesn't lag your computer at all. Only has a 78% detection rate, but catches the most harmful of viruses. If you're a normal home user, you're not in too much danger of higher level viruses and this is the AV for you:
http://www.free.grisoft.com
2) Avast! 4 AntiVirus. Smaller, sleeker skin, average scanning speed, but reports alot of safe files as viruses, so you might need to watch out for that. 78% detection rate, but alot more complicated than most other free AVs. Use this if you are handy with a computer and you know what needs to be scanned:
http://www.avast.com/eng/avast_4_home.html
3) Avira AntiVirus. Simple layout, which even the most basic of users can navigate. Incredibly fast scanning, with a 90% detection rate. Not as easy on the eyes as other AVs, and you need to know what you're doing to get the more advanced features, but it is definite a candidate:
http://www.free-av.com/en/download/index.html
Personally I use AVG, but only because I go for the easier interface. If you are paranoid about viruses then the only real option is Avira. But if you just want normal protection, then any of the three will do it for you.
For any spyware problems, get SpyBot - Search and Destroy:
http://www.safer-networking.org/en/mirrors/index.html
And for protecting your computer for malware or tracking cookies, use Ad-Aware 2008 Free Edition:
http://lavasoft.com/products/ad_aware_free.php
All of them are free, as powerful as paid programs, like McAfee and Norton (which only has a 18% detection rate!), and don't use up as many resources.
Just boot into Safe Mode (Press F8 when turning on the computer) and run full scans.
Hope that helps.