Windows only sees one core

I am running Windows 7 Ultimate 64bit. It has been running well, but recently I noticed that one of my cores from my core 2 duo is not being shown in the task manager, msconfig or speedfan.

In the task manager, there is only one graph, and the view>cpu history>one graph per cpu option is greyed out. In msconfig, under the boot configuration options the drop down list for the cpus to be used at boot shows only one cpu. In speed fan, there is only a "core 0" temperature graph.

I have tried clearing the cmos, this made no difference. Also, I have tried two Linux live cds which both happily use both cores. The interesting thing is that in the device manager it shows two cpus just as normal.

Does anyone have any ideas?

I will try updating my BIOS, it is not running the latest version. However, if the problem was this would it not be effecting Linux (Ubuntu) as well....

If updating the bios does not help, try a couple of things one by one:

1) In MSConfig > Boot tab > Advanced Options > uncheck "Number of Processors". Reboot.

Processors - Limit Number Used by Windows 7

2) Delete the processor entries in Device Manager, then reboot. Windows will now automatically redetect and reinstall them.

3) Download and install EasyBCD. Go into Advanced Options in EasyBCD and select "Limit Windows to '0' CPUs (Leave 0 to reset)". Click "Apply Settings" and exit EasyBCD. This will force windows to reset the count to default for the number of Cores you have.