This usually happens with motherboards that are not acpi compliant, but you said it worked before. Very odd.
To make it shut down, uncheck the APM under power management. You will still have to manually shut off the computer but windows will shut down. I realize that this is not the cure for the problem, but it is the best i can do.
Last resort would be to re-install win2k. Not alot of fun, but it should restore the original function. This will basically delete the Winnt directory.