My advice is to avoid Norton; there is better A/V software that doesn't use up all your system resources. One of my roommates earlier this year converted me to
NOD32. After seeing it in person, I don't think I'll ever use any competing virus scanners for the foreseeable future.
About 80% of NOD32 is written in assembly; this means that the program runs
insanely fast and uses only a few megabytes of RAM. It's also the least "obtrusive" of all the anti-virus software I've used, while still providing amazing "always on" virus scanning. And so far, I've haven't had it cause any problems or conflicts with any other software in either XP or Vista.
The only downside to NOD32 is that the user interface can be difficult to use at first. But after using it for a few days, it becomes no different than any other virus scanner.