I doubt its the screen that's freezing - its almost certianly the computer. Norton has a number of features to check the health of your system, so you can try running these. Officially, these also should warn you when the system is about to crash - unofficially if any thing, by the time the warning comes up its already too late. However, the most common problems causing crashes are damages drivers and damages software. The first thing to consider is what you did just before the problem first occurred. If this offers nothing, in XP you can go back to earlier set ups until you find one that does not have the problem.