I have tried just about everything and I have stuck with Softwin's Bitdefender (now 8.0). It uses WAY less resources than Norton (a real hog) and MacAfee. It has better virus protection than PC-Cillin, though I like the PC-Cillin interface.
I have tried Kaspersky and MKS_Vir (which now has a new name) because they got great reviews, but both caused problems on my laptop (memory leaks, errors, etc.)
In the final analysis, the best thing is to have SOMETHING and keep it up to date.
Also, a Spyware tool is essential as well. I have three: Spybot Search and Destroy, AD Aware and Spysweeper.
Finally, Firefox keeps a bunch of stuff off the computer. The amount of spyware that I had to clean off on my wife's computer went down by 20x after she switched to Firefox.