I've recently (as in within the past 4 days) been experiencing crashing on windows xp.
It crashes while running Firefox, while playing games, trying to set a desktop background, and even when sitting with no programs running!
I have installed a new back-up hard-drive, (Western Digital 750g) but that was almost over a week ago now.
Haven't recently installed a any new software.
Only thing I have added is a new case-siding fan, which was about 4 days ago.
And I say irregular because there is no set-time frame for the Blue-Screen, it could be anywhere from 10minutes to indefinite.