We've used Norton anti-virus for years on our prior computers. Had it on this one (we just got it Aug. 2004), bought the Norton anti-spyware upgrade this summer and it totally screwed up our computer. Had to totally uninstall Norton completely. Now we're using McAfee, and so far (3 or 4 months) haven't had any problems, knock on wood.
I think if you stick with one of the name brands, you'll do ok. I hear good and bad about all of them, but it's got to be better than no protection at all!