If you are looking to pay for one, I'd recommend NOD32 AV (not the the whole security suite). It is blazing fast and has one of the best, if not the best overall VB100 scores.
I have been testing the new MS Security Essentials AV program, and it is not slowing down my system at all. As long as the coverage is good, it could be a winner.
If you look at the VB100 tests NOD32 and Kaspersky are always scoring the highest. Norton used to have a lot of resource issues, but seems to have gotten better. The same goes for McAfee.