Windows only uses one cpu core

I'm running Windows xp media center edition 2005 with a core duo T2300. Device manager
reports both cpus, however task manager graphs only one.

www.rollscythe.com/images/taskmngr.JPG
I cannot set an affinity for any programs either. Cpuz reports the correct processor,
but the option to select different proccessors is disabled.

www.rollscythe.com/images/cpuzdvmngr.JPG
I've checked the bios to make sure its set to use both cores, which it is. I also
have Windows xp pro installed on a usb hdd which I booted from, it shows two graphs
in task manager as it should. So its not a hardware issue as far as I can tell since
one install of windows works fine using two cores. What would cause one install
of Windows not to use both cores and what can be done to remedy it? Thank you!

 

Under the taskmanager with the performance tab selected click on the view menu, cpu history. then select show one graph per cpu