Windows 7: Quad core processor only running on one core.

I'm having an issue with my CPU. It seems that it is only using one of its cores. This issue first became apparent to me when I was trying to run Blacklight Retribution. It popped up a warning message saying that my computer does not meet the minimum requirements of have at least a dual core. And indeed the game was unplayable, with single digit fps constantly.

However when i go to the performance tab of my task manager it shows 4 graphs, meaning that 4 cores must be running. Then I go the boot tab of my systems configuration, click on advanced options and underneath the "Number of Processors:" check box (which is unchecked), it shows 1 as the number of processors.

My processor is an Intel(R) Xeon(TM) CPU 3.60GHz (4 CPUs), ~3.6GHz.

These seem to be conflicting and I'm not sure what to do. Please help me.

This will answer it for you........
Processors - Limit Number Used by Windows 7

Unfortunately this doesn't help me. As I said before the "number of processors" checkbox was already unchecked, and checking it and changing the value below is only useful for testing purposes.