the computer was working Ok until this started happening? If that's the case, the cause is certainly likely to be a software conflict, and you can resolve it.
Roll the computer back to a steady state, using the Windows XP restore facility.
Then, once you're happy that things have settled down, install your software applications one at a time. When you install a program, let things run for a few days before you install anything else, and create a new restore point before each installation (if Win XP hasn't done so already). Name the restore points with the name of the program that you previously installed, so you know where to roll back to in case of trouble.
This method takes longer, but it's pretty well bomb-proof, and you'll spot the culprit immediately.