Definitely run Ad-aware, Spybot and one antivirus program. More than one antivirus programs running at the same time can cause problems. See (http://service1.symantec.com/SUPPORT...00031316555206).

Ad-Aware (www.lavasoftusa.com/software/adaware/)
Spybot Search & Destroy (http://www.safer-networking.org/en/download/)

For antivirus My choice is Symantec
http://www.symantec.com/product/index_homecomp.html

But there are many to choose from (Free and $$$) in no particular order:
McAfee (http://www.mcafee.com/), AVG (http://www.grisoft.com/doc/1), ClamWin (http://www.clamwin.com/) ... Just to name a few.

You can (and probably should) run a personal firewall if for nothing else than seeing what on your box is trying to make outgoing connections. ;-)

I like Kerio Personal Firewall
http://www.kerio.com/kerio.html

If you have a high-speed connection you can (and probably should) install a firewall/router. Like something from LinkSys (http://www.linksys.com).

-ts