Sounds likea problem in the motherboard's BIOS. If it's an older PC it may not implement all the latest power "tricks" that 2000 and XP are using.
When the PC boots, hit [del](most likely) to enter the BIOS setup screen, and look for a section having to do with Power Settings, see if ACPI(APCI?) is enabled. Might have to break out the motherboard manual or find it online to find what optimal settings are for you
Also, there may be some clues in windows itself- control panel -> display -> screen saver -> advanced is where most of the power options are, telling windows how to handle powering down, to use the advanced features
Check again with windows update, specifically in the drivers section, and in the windows(probably not i nthe critical updates section) updates, see if theres anything for power or ACPI