Antivirus.

[onmyouji]

Avast AntiVirus!
the home edition is free and very stable

[thyella]

I use Avira... I use it for many years and it's very light.
www.free-av.com

Same here and very satisfied!

[Kirie]

I have Norton 2005 but since I couldn't update it anymore, I downloaded AVG and... today I ran a full scan, first with Norton... it didn't detect anything.... then I ran AVG and it detected a troyan, the first thing my AVG detects! I supposse that makes it good (but my cd rom keeps opening by itself even though AVG deleted the troyan so now I don't know what's the problem )

[Munemasa Katagiri]

I remember there was an old trojan like software that could open your CD, make sure you don't have things like that, make a HijackThis log.

[Camysan]

Spyware Terminator for me. I tried several and this one has been a keeper. It's free and it's powerful.

-Real time shield
-Scans for spyware, malware, adware, and viruses. Fast scan is very fast.
-Scans through your internet cookies (a wonderful help since that sometimes screws up my internet), favorites, and you can immunize your computer to protect it.
-Includes a toolbar for your browser (asks if you want it). It tells you if a site is bad and asks if you wish to not go there or to continue. One other thing is that it blocks ads that have adware.
-More here