Broken Windows (XP)
A cousin's computer died. It got a BSOD upon boot. None of the boot methods worked (e.g. safe mode, last known good config, etc.). It turned out to have a corrupt ntdll.dll file (which is reported with a "STOP: C00000221 UNKNOWN ERROR" BSOD), among other problems (it turns out the HD had some bad sectors). I learned a handy trick for fixing Windows problems like this from Rick @ TechSpot. Basically, it involved telling Windows you want to install, then accept it's offer to repair. (This is instead of using the standard repair option.) That page also had a handy link to lots of HD utilities.