Upgraded CPU to X2 - doesn't show in control panel

I replaced the processor in one of my desktop PCs with an Athlon 3800x2. In control panel (xp pro) it shows as the correct processor, but clicking on processors and also in performance monitor only shows a single CPU.
I downloaded the amd drivers and tried them, I've also done a repair install of XP with no success.
Anyone got any ideas ?
thanks simon.