If you're gonna purchase any buy Kasperskys .......... supposed to be the best around with an alleged 98% of 'nasties' detected , that's about as good as it gets ........ there are one or two other good ones , but Kasperskys lays claim to be the best.
AVG is a very good free one (about 60% of 'nasties' detected , it's not perfect but it's adequate ........ Superantispyware is a pretty efficient one too , although it's not a free one , as is Spyhunter ....... McAfee seems quite popular , although i haven't tried it myself .
I found Norton to be a waste of time .
Scout around for one though on the net ....... some offer a months free trial , try before you buy ....... nothing is perfect , but you do need some kind of protection.