always automatic restart the computer

i had problem with my home PC.it’s always restart by itself while i’m using it.below its my pc specification.

SYSTEM : microsoft windows XP professional
version 2002 Service Pack 2

COMPUTER : Intel(R) Pentium(R)4 CPU 2.40GHz
2.30 GHz, 512MB of RAM

MOTHER BOARD : VIA PT800/P4X533 series
ASUS

DISPLAY ADAPTER : MSI RADEON 9550

can someone give answers & suggestion why my CPU always restart by itself.