Counter Strike 1.6 Keeps restarting my pc!!!?

When i launch cs 1.6 i could play for 3 minutes at least. then it restarts my computer. i just got off of a game that lasted about 10 minutes the it restarted my pc , the longest it has ran. Whats wrong? It does it with half life and cs zero! but it dosent do it on quake 3....