Why my computer restarts?

my problem is this: the computer restarts after a strange noise comes from the hdd, like it stops and starts again. sometimes i can hear how the hdd crashes, and sometimes the computer restarts without no hdd crash. the thing is that everything is ok when i don`t do heavy stuff on my computer. but if i wanna play a warcraft game on garena, as i enter the game the hdd stops, starts again and then the computer restarts. sometimes the hdd can stop and restart 5 times before the restart. i`ve cheked the hdd with drivesitter. everest and another prog an none said is a problem with it. so what is?? my hdd is a western digital sata 80gb, on a asus p4p800se, 2400 intel pentium 4, 512 ddr1 and ati radeon 9600 pro.