Windows Nt power down after shut down

Recently I bought an Asus mainboard to set up a NT workstation. The workstation is required to boot up and power down on its own at the required time.Booting up at the desired time is not a problem since the bios can be programmed to do so.
My problem now is that when ever the workstation shuts down, it does not power down but reboots. I even tweaked the registry but still the same.
So what is causing the problem?
I noticed the power down function works well with windows 98, using same OS and same casing.
Can it be the casing? I am using an ATX casing though. Or do i need any kind of NT patch?
My system info :
processor : PIII 550
mb : Asus P3V133
OS : Windows NT workstation 4.0 (sp4)

Please advice.