Power Supply won't shut off

I upgraded my main PC recently, so I bought a cheap case with a 400W Powmax PSU to build a second computer using the "old" parts. The problem is the power supply does not shut off, the "shutting down XP" screen just stays on the monitor. I can hear the little click that usually occurs when the OS tells the PSU to shut down, but the PSU just keeps humming along. I have to manually turn off the PSU.

The MOBO, proc and memory worked fine together in my old system, so I'm assuming it's the new PSU. ACPI and APM are enabled in the BIOS.

I've never seen this problem before. What could be causing this problem? Is there anything I can do about it?

Thanks.

"There's no such thing as gravity, the Earth just sucks."

If you mean the power button on the PC, that doesn't do anything. I have to use the power switch on the PSU to turn it off.

"There's no such thing as gravity, the Earth just sucks."