From what i'm seeing, i think that there must be something wrong with your operating system. This can be caused by something that might have been installed even with or without your knowledge. Since you said that there is no trace of spyware, it could be something you did yourself. ---- Now don't be too panicked.
First, I don't think it has to do anything with your video card. That could cause your system to restart.
My guess would be a major change in the Operating System. -> For you to have more than one program open, requires more Random Access Memory (RAM). The OS could have a problem managing RAM or even system cache.
What could be a major OS change? - A windows update or Service Pack 2 which makes adjustments to your System files and Sytem Registry.
The Solution: Since you have Windows XP, you also have System Restore. System Restore brings back your OS to a state before you installed/uninstalled a driver, update, game or even a simple program. So go ahead and click to Start > All Programs > Acessories > System Restore.
Choose an appropriate time just before you had this problem. (You don't want to go back in time too far as you would lose some programs) Notice that this will just turn back the state of your system. It will not cause you to lose any files or documents. And again, this operation is 100% reversible.
I hope this helps, Good Luck