system automatically shut down

i have a system with the following configuration

Pentium III 534 MHZ
64 MB RAM (SDRAM)
20 GB HARD DISK
XP PROFESSIONAL AS MY OS(ONLY ONE OS)

THE SYSTEM WILL AUTOMATICALLY SHUT DOWN AFTER STARTING OR SOMETIMES IT WILL RESTART AUTOMATICALLY (THIS WILL HAPPEN EVERY 10 T0 15 MINUTES) WITHOUT ANY ERROR MESSAGE

SOMETIMES ALSO IT WILL NOT BOOT IMMEDITALLY AFTER I PRESS START BUTTON , SO I HAVE TO PRESS 3 TO 4 TIMES

AND SOMETIMES THE SYSTEM WILL WORK FINE.
AND SOMETIMES IT WILL RESTART OR SHUTDOWN AFTER LONG HOURS LIKE 1 OR 2 HOURS

BY THE WAY WHEN THE SYSTEM AUTOMATICALLY SHUT DOWN THE RED COLOR OF THE HARD DISK WILL STILL BE BLINKING AND ALSO THE CPU FAN WILL BE STILL ROTATING

WAITING FOR UR REPLY