What could cause a PC to keep resetting without even POSTing?

Changed son’s innards over to new case. My old one and it worked fine for me.

Same PSU from his old case, same everything. I did take the CPU off, remove old paste and reapply some Arctic Silver 5.

Anyway, when I turn it on, it powers on then resets about 5 seconds later. No video output at all, no beeps. Checked behind board for something causing it to short, nothing. Did a barebones startup, no difference. Tried unplugging the power switch from the board, no difference. In desperation I unplugged every front panel wire from the board!

Any help guys?

EDIT: forget it, I took the CPU off and reinserted it as ell as shook the case all over the place and banged it against the floor several times in case I mi8ssed something that was lodged on/between the mobo. I broke off the USB flap cover, but it works. :s

This sort of problem with fast resets is usually down to CPU heat sink not seating correctly causing the CPU to overheat, shut down and restart as soon as it is cool enough.

Since you’re not even getting to POST, it’s either memory, CPU or vid card. Also check your CPU for a bent pin.

Sounds to me like something is not plugged in from the PSU. Check and re check all the cabling.

Thanks guy - probably was the CPU then. Good to know for the future.

(Bob, see edit in my post above)

ha ha ha…nothing works like a good toss across the room or bang it with a hammer!