for windows 7+ i recommend either MSE or ESET NOD 32
for winxp AVG
MSE can be a bit uncontrollable at moments but it is freeware and since it becomes integrated in win8 i think MS will invest alot in it
I personally used ESET for over 5 years and i was happy with it, now using MSE at home and AVG/MSE at my work pc`s