Desktop won't power down

Here is a novel problem I've not run into before:  my desktop won't power down at the end of an O/S's shutdown sequence.  I have an older P3 (circa 2001) Compaq desktop that I use as a lab pc.  I refurbish p4's for a local charity, and I use this unit to test hardware, drives  and applications.  I swap drives in and out by simply changing IDE and power connections to other drives.  I never have more than one IDE drive running at one time.  The BIOS has been very good at detecting the new drives, prompting for the changes, and so on.

That's just background as to what it's been used for.  I routinely swap DOS, Linux Win2k and Win98 drives as necessary.  So far no problems.

Recently the system won't power down.  At the end of the sequence, the power remains on and the system reboots.  It's not the OS, since it does the same with Linux and the various flavors of Win I swap in.  I assumed it was the power supply.  But I swapped in an identical P/S and - surprise - same behavior.  So - it's not the OS or P/S, so mobo, BIOS?  I haven't changed any jumper settings or otherwise altered the board.

MOBO specs:

Manufacturer      Compaq
Model      0684h
Serial Number      V108DYSZA646
Chipset Vendor      Intel Corporation
Chipset Model      815/EP/P Solano Host-Hub Interface Bridge (GMCH/MCH A2-step)
South Bridge      801BA LPC Interface (ICH2 B0 step)
CPU      Intel Pentium III
Cpu Socket      Socket 370 [XU1]

Right now the only power down option is yanking the cord from the P/S when the power.  Kinda unhandy, and probably not great for the system.