welcome to the Microsoft World. BLUE SCREEN OF DEATH. this happens due to many reasons, one most important is software/ hardware incompatibility. if you have recently added some hardware or software??

try restarting in safe mode and share what happens, you can copy your important files to d or e drive (if c is OS drive)

if you have original CD/ DVD run the setup again in recovery mode (sometime it helps)  (XP/ Vista what do you have?)

if you can access some of your friends computer download Ubuntu (linux) live CD and burn it and make booting CD. then Boot the system from ubuntu live CD and move your files/ folder to any other drives such as D and E if windows is on C. then, you can format C and reinstall XP/ VISTA.

good luck