There may still be some values left over in the registry from when you pulled Norton's from the system. Best way to get rid of those is to use
registry mechanic. Don't try and remove the Norton's software keys yourself unless you know what you're doing...otherwise you can crash the computer.
Another thing which maybe a problem is a corrupt video driver. Go into safe mode and reload the drivers. Although sometimes they just play up, so one way of fixing the problem in safe mode is to disable the screen and then allow XP to reload the drivers on boot.
I've never liked Norton's stuff. It has the nasty habit of taking over your computer and when something goes wrong, it either locks you out or stuffs something up.
Another problem might also be a corrupt boot file (boot.ini). Here's the
MS KB page for the boot.ini file and how to repair it. Try
here as well. It could be a problem with your partition table or MBR, in which case open in safe mode and then go Start>Run>fixmbr. That'll rewrite the MBR and hopefully cure what ails you.