one of the major causes I have found is forced M$ updates which fail to fully install.
The first time I encountered this was on one of my own computers running an OEM install of XP.
Some update came though, but failed to completely load, causing the BSOD, I managed to force the system to uninstall about 10 updates until I got rid of the BSOD ... but then suddenly the registration timer pops ups stating I have 14 days to activate the OS !
Next re-start of the computer the timer maxed out and locked the computer. After trying almost everything possible to re-activate, I gave up and contacted M$ support ...I was on the line for over Two and a Half hours trying every combination of codes the tech could dig up, and in between, they kept asking if this OS had been installed on another computer previously, to which the reply was it was a factory install on that machine [ a Toshiba Notebook ] ... we eventually hit on an activation code which worked - and of course I wrote it down for future reference - which I did have to dig up another time ... ... ...