I had a similar problem but it was with an Intel Core 2 Duo. It was a long time back now but the problem I recall was the fact that ACPI was not enabled in the BIOS. It all to do with the HAL (Hardware Abstraction Layer). If you are running Vista and you change the BIOS to enable ACPI and boot up you will then see miraculously both cores running. If you are running XP then it could be that you need to run some other software to fix the HAL or in the worst case re-instally XP after you set the BIOS to enable ACPI. Search on the internet for HAL if you want any more info as my experience was a long time back.
I can't seem to find ACPI at all in my BIOS.
Go to System properties, in the advanced tab go to environment variables. Check the variable NUMBER_OF_PROCESSOR. If it is set to 1, edit it and set it to 2. Then restart the system.
I tried this, but once I restarted, it was set back to 1.