Randon Resets

OK, I recently built my PC, and when I initially had everyting in, I went to run it and it didn't run, the psu was dead.  Sent it back got a new one runs perfect, I think.  But now I have random resets every so often even when I'm idleing.  So if you could help me out and help me figure out the problem that would be great.

Blow on your processor, check for dust build-up. Tell me what happens.