When I hit the sleep button in Windows the computer looks like it's going into Sleep mode but a few seconds later it's restarted, and stops with this error message:

internal hard disk drive not found
To resolve this issue, try to reseat the drive.

No bootable devices--strike F1 to retry boot, F2 for setup utility
Press F5 to run onboard diagnostics.

The computer is running Windows Vista (SP2) and I have installed all available Windows Updates and the latest manufacturer drivers.

I have already tried to reseated the drive, ran the onboard diagnostics and there were no errors.

I have changed the power settings for all devices where it's available so that they are not allowed to wake up the computer.

Any ideas?