Windows xp cannot power down computer

Hi to all of you!

I've installed Windows xp and everything is working fine, except for the
shutdown procedure. After shutting down the system, I get the message 'It is
now safe to turn off your computer" and I have to press and hold the power
button for about 5 seconds in order to turn off the machine (one brief press
doesn't work!).
I enabled APM in the power options (thus tab APM is available), but this
causes the machine to restart!

The best thing would be to turn off the machine automatically, the seconds
best thing, but still acceptable, is the "it is now safe to turn off
"-message after which you have to press the button briefly.

Here's some more information:
- award bios v4.51
- amd k6-400 Mhz
- windows xp
- motherboard? I don't know, how can I figure that out?

Does anyone know a solution for my problem?

Thanks in advance!