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WAIT!!! Don't reinstall just yet!
I had this problem too, just 2 weeks ago. For some unknown reason, Vista (mine is 64 bit) did not pick up the newly installed Q6600 and still thought I had an E6400 in place. Funny, that XP picked up the change just fine on my testing rig (I boot to another hard drive with XP loaded for testing).
What I did was went into Device Manager and uninstalled each instance under the processor tab and restarted after each core had been uninstalled. At the next startup, it recognised the CPU change. Was really weird.
Something for Microsoft to fix. Don't go resintalling just yet. Persevere and keep plugging away - I may well have experienced more luck than judgement but I got there in the end. Might be worth doing a DMI clear on top of the CMOS clear. I would take out the battery for a couple of hours to completely reset the BIOS and see if there is an option to clear the DMI pool (and/or ESCD).
/NINJA EDIT - I was too late.
