computer self-restarts

[Likwid_Nitrogen]

My computer self-restarts about 3 seconds after power down everytime unless I throw the switch on the power supply. I only have to turn off the power supply during that one 3 second moment; I can turn it back on immediately after without the computer turning on. This only happens after a normal soft off, and not if I hard off the system (holding the power button 5 seconds or whatever). Also happens if the system tries going into suspend mode, putting it into a zombie-like on-but-won't-wake-up state.
All wake-on-(x) options are off in bios. Problem persisted before and after a new case build. In the old case build, I would hear an audible click from somewhere at the restart moment 3 seconds after power off, whether or not I had turned the power supply off.

Any ideas?

 

[Dangman]

Please list out the specs for both builds. Also, are you using any part from your old build in the new build?
Posted via [H] Mobile Device

 

[Likwid_Nitrogen]

Ah yes. Same components in both builds, just a new case.

Asus PQ5 SE/R mobo
Core 2 duo E8500
2gb patriot
Asus TOP gf260
500gb Samsung Spinpoint F1, 1tb Samsung Spinpoint F1, 320gb WD
dual boot XP / Win7 (happens either way)

all currently shoved in an HTPC case.

no overclocking

 

[Dangman]

What power supply?

Be sure to hit up the stickied basic troubleshooting guide.

 

[Likwid_Nitrogen]

SeaSonic M12II 500w