Shutdown problems - troubleshooting suggestions:
Computer restarts instead of shutting down from inside Windows
Change the registry setting: HKEY_LOCAL_MACHINE\software\microsoft\windows NT\current version\winlogon Change PowerdownAftershutdown from 0 to 1.
Have you checked your BIOS power settings? Make certain the "Wake on Modem Ring" and "Wake on PCI PME" are set to "Stay Off." If they are not, change them and see what happens.
Check if any hardware updates have been made recently (I.e. adding a wireless NIC that could be set to wake on connection; a firewire card, etc)
First, make sure your Advanced Configuration and Power Interface (ACPI)-enabled computer has an ATX case. You can usually tell this by looking at the back of your computer on the power supply. If the power supply has only one plug and a small switch on it, the case is probably an ATX case. If you don't have an ATX case, you'll have to find out how to remove automatic shutdown (which turns off power on ATX computers).
Second, download and install Windows 2000 Service Pack 1 (SP1), which includes an updated version of hal.dll, the culprit file in the shutdown issue. You can get SP1 from the Microsoft Web site.
After you download and install SP1, the computer will attempt to restart. If this fails, wait until the computer presents a black screen and you see no evidence of hard disk activity. Then, press the reset button on the computer (or switch the power to it off and on). When Win2K starts up again, all should be well!
It could be the power supply