DO NOT USE McAfee!!!! Ive had so many problems on peoples computers who use McAfee! It is one of the worst anti-virus programs out there. They don't pickup anything and they hog system resources and cause weird slow down problems.You use Comcast? They have free McAfee.
this is what I am using, and so far i have no complaints
it was suggested by robvas (thanks)
http://www.microsoft.com/Security_Essentials/