A clean install of XP without being tweaked uses anywhere from 130-250 MB of RAM just for itself and its services it loads up. Add to that 3rd party software and the number only rises.
I have seen PS use 80MB of RAM just from letting it sit at the login screen.
Since the person has stated they know little about thier computer, I am going to assume they have also not tweaked the page file settings. By default, XP sets a page file settin gof 1.5 times the physical amount of RAM in the system.
So it is quite possible that with only 256MB of RAM, after having PS open for a little while, what with its memory leaks and all, it does run the system out of RAM. At this point the computer restarts, as that is what a windows OS does when it runs out of RAM.
http://www.techspot.com/vb/all/windows/t-1937-Normal-RAM-usage-with-WinXP.html You also have to think about the video card. It only has 64MB of onboard RAM. When that fills up, it turns to motherboard RAM, further increasing the load.