my onboard graphics works fine but everytime i put my graphics card in then it does the same thing...so i gues its my graphics card thats "dead" ...im going to buy me a new graphics card so mmmm thanks guys for the info...i now know something new...
no no no... as smorizio said, it could be your psu. Don't be so quick dumping you gpu. Usually when the gpu kicked the bucket, you will see artifacts on your screen, but will not causing your system to restart.
Your onboard's fine because it drew less power from the psu, that's why it run ok.