I currently have XP with SP1 and when i download from microsoft.com sp2 and installed it on my system, it just said restart your system and then it wont boot anymore.
OK , so if you turn the power OFF to the pc
then turn the power on , do you see anything at all on the screen, like a mainboard logo or bios info...etc. ?
also make sure there is nothing in your cd/dvd drive, no memory cards in card readers etc.
did you say this was a desktop or notebook, what make and model