2nd hard reboot always required

My ASUS P4P800SE running WinXP has started to require a 2nd reboot (using the front power button) after any hard (e.g. power failure) or soft reboot in order to properly start running again.

Example: use Shut Down->Reboot from the GUI. System will close gracefully to a black screen, but when the disk light starts blinking at the start of the reboot, my monitor will go to standby mode, and the system will never reach a working state (I have apps that update a web site every 30 mins, and if there's been a power failure during the night, I won't see an update after the power failure time - but the system is humming away with a monitor in standby).

I can always restore the system by holding the power button on the front of the case in until the system shuts off, then powering it back on. I've never needed to cycle the PS power switch on the back of the case.

Using the reset button to restart the system after it's been running will also require a power cycle using the front power button.

I've no idea why this is happening - any ideas on how to fix /debug the problem are welcomed.

Thanks,
Rob