computer (XP) restarts by itself?

Question

and when it restarts there is a black screen thats has the options:
SAFE MODE
SAFE MODE WITH NEWTWORKING,
RESTART WITH LAST KNOWN CONFIGURATION SETTINGS
RESTART NORMALLY

it restarts every 15-20minutes

Answer 1

check your event log for any information. this can be sue to a hardware failure sometimes faulty memory can do this

Answer 2

have you installed any new drivers. then try the restart with last known configuration settings. or the problem with hardware malfunctioning.