Well everyone has shelled out their hard earned cash on a antivirus product at some time or another. Well no need to do that any longer there are many free options out there these days
avg for instance http://free.avg.com/us-en/homepage
and MS security essentials http://windows.microsoft.com/en-US/windows/products/security-essentials
I personally use and recommend MS security essentials for simple reasoning. MS makes it so there is a much slimmer chance of problems between os and antivirus as well as windows performance degradation due to Anti-virus activities. If you have ever worked or used a machine running the full Symantec or mcafee security suites you know what a performance hit a machine can take from such products. and you get the added bunus of paying 50 dollars or so a year to have your machine take said performance hit.