Will some Expert in computer please help me solve this problem

[handsomeness]

i brought this computer about a month ago. At first nothing happen, but now it start to show some problems. When i play game(counter strike), sometime the computer reboot. it also reboot and shutdown automatically when i surfing the net. my motherboard is MSI neo platium 250gb, 516 mb, 400Walt power supply and window xp...my cpu is amd 64 3000+. SO please help me....

 

[mechBgon]

What brand is the power supply, is it a good brand like Antec, Enermax, Fortron or Sparkle Power? Quality can be important.

 

[jackschmittusa]

Are you monitoring the temps? Changed any drivers lately?