My pc turn off automatically please help!!

Hello,
So,here's the problem
i've that i play online
games [gta samp] and i
don't have graphic card
for it but also it work
fine.when i play it turns
off automatically and
then beeps [ countinous
beeps ] and it won't stop
after some hours when i
turn on the pc the beeps
are no more...like that it
goes shutdown again
and again and beeps.
Any help will be greatly
appreciate.