I'm thinking about upgrading my current motherboard, but I know that windows will recognize the hardware change and want to be re-activated. Can I use the same key that I had before?
If it's a vendor (Dell, HP, etc.) installed version then no, you cannot reuse the key on new hardware. Any other version, as long as it is only installed and activated on one PC you may use the key to reactivate. Microsoft has relaxed the rules (but only with Windows 8) with regard to transferring OEM licenses.
Best to call Microsoft but I'm guessing that it wouldn't work. I haven't tried to do the whole old key new computer switcheroo in....about 10 years though so I might be wrong.
If it's a vendor (Dell, HP, etc.) installed version then no, you cannot reuse the key on new hardware. Any other version, as long as it is only installed and activated on one PC you may use the key to reactivate. Microsoft has relaxed the rules (but only with Windows 8) with regard to transferring OEM licenses.