Pc shuts down while gaming.

Hello,
pc shuts down automatically when i play gta sanandreas..

when i install new windows then it works but well after some days of installing new windows the problem occurs again...and when i again install the new windows and it works well but again after some days of installing new windows it again shuts down while gaming.... whats the problem

ram>> 2gb
hard disk >> 500gb
processor >>> intel 3 giga hertz
i dont have graphic card;;

<<<small games works well but big one like gta vice city, gta sanandreas, nfs most wanted get stucked and then pc shuts down>>>.

//////////////////windows 7 < 32bit >/////////////////////////..
thanxx