couple of things:
1) i had a m/b crapout on me and swapped the HD with OS over to another system. (single proc to dual core) The swap worked post and boot, but pulling up taskmanager it showed only one processor. There may be some registry entry for the number of procs, so it may not autodetect the number of cores in a cpu swap out.
2) i had to get a new m/b with a different chipset to replace the bad one. The process isn't overly complicated but it is a pain, a lot of popup and dialog box clicking. You have to remove all the drivers and use some "show hidden devices" options to find them all. The thing is, you're bound to miss a few entries/drivers. The audio chip on the new board still seems a little glitchy. I might have better luck with a clean install, but this is the price i pay for keeping my hd the same. So it works but may not be perfect.
[The article is somewhere on Arstechnica.com "How to install a new motherboard without reinstalling Windows" by john stokes.]
Let us know how the swap goes.