Avira offers a free anti-virus package that blows away many commercial and older free products.
ESet offers free one-shot anti-virus removers and a strong anti-virus commercial offering for schools. Again, a product that is way ahead of the pack.
Finally, with the disclaimer that Alex Eckelberry is a personal friend of mine, I am adding Counter Spy to my recommended security suite for parents, students, and teachers.
Don't take my word for it if you think I'm selling something. I make not a penny and I believe this will secure your computer better than just about any other affordable suite of software.
My links are on the left and the updates are worth doing now.