Well..! I found this in one of the forums:
To change from ACPI Single To Multiprocessor PC.
* In Device Manager navigate to "COMPUTERNAME/Computer"
* Right click on "Advanced Conficuration and Power Interface (ACPI) PC" and click on "Update Driver"
* Select "Install from a list or specific socation (Advanced) and click Next
* Select "Don't search. I will choose the driver to install" and click Next
* Untick the "Show Compatible Hardware" box and select "Advanced Conficuration and Power Interface (ACPI) Multiprocessor PC" (1) then click Next
* Follow the wizard through untill reboot. Should be good now.
(1) If you do not see this option to select, you may need to manually force windows to think you have 2 CPU's. Listed below is how to do this.
To force windows to think you have 2CPU's.
* Right click on My Computer and go to "Properties"
* Click on the "Advanced" Tab
* Click on the "Environment Variables" button
* Look at the "System Variables" box.
* Make sure the "NUMBER_OF_PROCESSORS" variable is set to 2.
* Reboot if needed.
Hope that helps. If it doesnt work, reinstalling Windows with hyperthreading enabled in the bios will work.