Seconding the recommendation for Avast. I used to use AVG but switched to Avast because it seemed to have a smaller footprint and be less of a resource hog. Avast has caught several potential nasties before they got to eat my computer.

Download it here (http://www.avast.com/index), be sure to choose the free antivirus - the additional features of the paid versions are overkill for most people. If you want the "data shredder" feature (military-grade data erasure) look for CCleaner (http://www.piriform.com/ccleaner), multiple-overwrites are an option under its "Drive Wiper" tool and can be applied to "free space only", i.e. the space from which you just deleted whatever data you want to "shred".