Computer does not remain in standby

After erasing Vista and formatting hard disc, (new computer), XP Pro has been installed with success. However, the standby function does not work, the computer would wake up after about 5 seconds for no reasons. No mouvement of the mouse , no background activity is enabled. The same applies to all BIOS settings.
What could have gone wrong?
A missing driver perhaps?
Thanks for your suggestions.

Try the settings under control panel, otherwise you just hit the on button to get it working again

Sometimes wireless mouse/keyboard might cause that issue.
If you do use something like that unplug it and try again...

Thank you all, but it was a missing driver that caused the problem.
PetervM