This is one of the reasons I wouldn't touch Norton Anti-Virus or McAfee. For starters Norton doesn't half slow your system down to a slugs pace, and McAfee... well that won't pick up a couple of Trojans that Clam Anti-Virus (which is free and open source) will pick up.
On Windows I tend to use AVG, sure it's a little slower than some but it works and works well. Also never had any problems with NOD32 either.
I dunno, I remember when Norton was a brand you could trust (Norton Utilities especially) and Peter Norton was well respected, but it seems like his name is being used to sell any old crap these days.