With the recent malware script. I thought I would share some free options that work to protect your computer.
Free Antivirus for XP, Vista, and Windows 7 32bit and 64bit - Avast, Avira, AVG, and Microsoft Security Essentials
Avast Free - http://www.avast.com/free-antivirus-download
I use Avast free. It detected and stopped the malware script from executing. It's free for a year, as long as you register it. After the year, just register it again.You don't have to provide your real contact information if you don't want to. You can fill out just about anything, and it will register just fine.
Avira Free - http://www.avira.com/en/avira-free-antivirus
Avira is another popular antivirus. It has a nag screen that comes up to buy it, but nothing major.
AVG Free - http://free.avg.com/us-en/homepage
AVG would be my 3rd choice for protection behind Avast or Avira.
Microsoft Security Essentials is a free download and highly rated. http://www.microsoft.com/security_essentials/
There is also a free online scanner from Trend Micro, a very reputable antivirus and Internet security company.
You can run 1 of these scanners together with the free version of Malwarebytes Anti-Malware http://www.malwarebytes.org/
If you use torrents or other peer to peer programs, then get PeerBlock. Peerblock can't completely protect you, however... it will add a level of protection while your file sharing.
FAQ explaining what Peerblock is and does http://www.peerblock.com/docs/faq#what_is_peerblock
Download page http://www.peerblock.com/releases
Even if you just use the free online scanner at Trend Micro, it will help go a long way to keeping you virus free.
*Edit* - I forgot about Spybot, good program. Good call Josh. http://www.safer-networking.org/en/index.html