Was it grayed out? If you tick the box (as Joe suggest below) then the drop down only shows a single core?
Yes, it was grayed out, however I went into my Vista 64 bit MSconfig and was actually able to change the Windows 7 boot settings from 1 to 4, yet when I started windows 7, only 1 core was working.
Have you installed any device drivers
(such as for your chipset) and / or stuff that might be 'legacy drivers / drivers that may have issues with Windows 7?
No, I didn't install any drivers post windows 7, it was all transferred via upgrade from windows vista.
Do you know of any Legacy drivers in particular that might be causing the issue?
Also, it installed on my harddrive.