You didn't tell about the machine, it's parts, the power supply so I can't tell if you undersized the power supply or if you should pull the case cover to see if it helps. Some new builders are unaware that you need a heatsink on the CPU!
It goes like this:
Turn on the comp and after a while it restarts, I hear a sound from the case of the hard disk shuting down. I thought it was just in games and that the problem is in my new grafic card (5 days old, the problem started yesterday) but it happened after or in the midle of an instalation and so on! I described the sound that the hard disc makes, and based on that I thought that it is a power box???( the thing with a went i dont know the english word) problem. Maybe the new card requires more power causing the lack of power in the hard disc????
Please help me!!!!