I've never had any trouble with Norton - use it at home, and use Corporate edition on the server and PCs at work. Maybe I've just been lucky... but, in my experience, everything Symantec sells does exactly what it claims to do.

I'd heard a few nasty rumours about McAfee doing strange things, but nothing definite... just rumours.

Norton and McAfee are the market leaders, so I don't really see how either could cause major problems, as they've both been at it a long time. Maybe it was a dodgy installation - Norton won't allow the 'Liveupdate' function to work unless you are using a properly registered version, which often means the virus definitions are out of date and useless.