Is the BSOD a flasher, or is a persistent one that stays until you shut down or it restarts after a memory dump? Either way, try and see the exact cause. There should be some text midway down the screen that tell you what the problem is. Only other way is I think, pending the version of Windows, to check the error console, which is accessed through the Administrator Tools.

XP (Home or Pro), Vista, or 7?