McAfee has a generous offer - head over to http://www.mcafee.com/myapps/mfs/default.asp for a free online virus scan.
I’ve got Norton AntiVirus running on this machine, and even though I pull down the updates and do a full scan weekly, I run a McAfee free scan every so often, sort of like going to another doctor for a second opinion.
Just watch out for the pop-ups. A little pop-up vigilance is the small price you pay for a free virus scan.