I just learned this and boy, what a difference! Now, most technical people may already know this, bit I'm not a complete computer noob and I didn't, so it's worth mentioning.
Apparently, when you install Windows 7 on a HDD, it records all kinds of stuff regarding your hardware on your HDD. If you install a new motherboard, this can create some serious resource conflicts making all kinds of components either not work at all or be very slooooooow. You may think, oh, my new MB stinks! Nope, not at all.
The short answer is: install a new MB, re-install Windows 7. It will work a charm - promise.
Windows has always been like that.
"Frankenstein" installations (hard drives transplanted from one motherobard to another, especially when they have different chipsets) has always been A Very Bad Idea. Actually, you're lucky if it even boots at all - usually, it'll bluescreen, because the HAL that was created on installation won't work on the new hardware.
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I just tried this recently for a giggle. Went from a nvidia nforce4 board s939 /w amd x2 3800, to a amd 790x with a phenom II. All I had to do was reactivate after win7 loaded the new drivers automatically. (altho I did run drive sweeper after to get rid of all the nvidia junk). Still running it now and won't reinstall fresh until we get the next public beta, if we do, or if something goes haywire but it's been running fine since Monday.