New Ga-nf7-rz mind of its own

I have just built a budget system for a work colleague using ga-nf7-rz
board, Athlon xp3000 and 512 mb (generic mem).
Second hand graphicard, using onboard sound ,pci dial up modem.
Windows XP with SP2

Runs ok , everything loaded ok, but turn it off, watch it power down, all
goes quiet, go away and make a cup of coffee and it turns itself back on. Or
worse, its turned itself on overnight or while out at work.

Any ideas how to overcome this, or is m/b screwed ?

Thanks
John