The scan’s mentioned by Solai are good one’s for Virui (viruses)
when I am scanning for Trojans or Maleware, in addtion to spybot search and destroy I use
malwarebytes anti-malware http://www.malwarebytes.org
ad-aware by lavasoft http://www.lavasoft.com
I also run CCleaner http://www.ccleaner.com
ccleaner will remove all the temp files and craaaaap on your computer so that the scanners don’t have to scan thousands of extra files…FWIW
From what I hear (and I don’t currently use Windows, so take it for what it’s worth) Spybot is as good as the big boys, and has an obvious advantage.
Pike, You are correct, From a certain point of view. 
Spybot only scans for maleware. but thas ok cause thats what the problem is most of the time.
but since they can only update their defintions once they learn of a threat I use the best three, to make sure I am looking for everything thats current.
the Malewarebytes updates like three times a day… WOW
I use AVG Free version…doesn’t hog resources like the big boys and scans for malware too.
That page was one of my main go-to’s for getting stuff fixed back in my consumer tech days. Very useful, everyone running XP should bookmark this page.
Virus Protection: I’d use AVG Free, Panda or Kaspersky. Keep clear of Norton and McAfee, they’re too much bloat for their own good.
Adware: Ad-Aware, Spybot, and Malwarebytes Anti-Malware
General Security/Stability: Upgrade IE, even if you don’t use it! At the very least have 7, if not 8. IE6 is so full of holes it can’t be trusted, and certain apps can get it to run even if its not your default browser.
I use it to do tech support for family computer issues…luckily I don’t to it at work anymore. Another great site for help that I use often is Geeks to Go! They have a forum that you can search for answers to your issues with just about everything…not sure how much Apple help since I don’t have one.