Computer Will not shut down properly

[Meaze]

I recently built a new PC and I am having an issue. When I shutdown the PC through the Vista Shut-down option my computer goes through the normal "Shutting down screen" but it frequently does not shut completely down. All fans are still running and the Optical Drives are powered. Their is no hdd activity. Has anyone got any idea what might be causing this? It will eventually shut itself completely off. Sometimes within minutes but often it takes hours.

Things I have eliminated:

1)It is not in Sleep/hibernate mode
2)It is not the mobo
3)It is not my ram
4)It is not the HDD
4)It is not Vista or any of the updates

My system:

ASUS P5Q Pro LGA 775 Intel P45 ATX Intel Motherboard
Intel Core 2 Duo E7200 Wolfdale 2.53GHz
OCZ StealthXStream OCZ600SXS 600W
mushkin 4GB (2 x 2GB) 240-Pin DDR2 SDRAM DDR2 800 (PC2 6400)
EVGA 512-P3-N801-AR GeForce 8800 GT 512MB

Thanks in advance

 

[shadowduck]

Have you applied all updates?

It is a known bug in Vista but has been patched.

Also make sure ACPI is enabled in BIOS.

 

[Meaze]

I have installed all the updates. I will check the ACPI when i get home. What is the ACPI? Thanks for the response