Have you always had these problems on this system?
As it occurs in both os's, it seems like a hardware problem, maybe like zipzoomflyhigh suggests, the ram.
Try running a memtest 86+ ram test. Goto their site and there you should find downloadable content to make a bootable cd or floppy (floppy is easier but I guess you don't have it anymore)
No, It does not occur frequently but almost once in a day in XP but in window 7 it happens when I just open a big file or playing games. Is it may be any problem with os or its updates.