Microsoft will NOT do that. The OEM serial number and activation only works with OEM software, which Microsoft will not sell to individuals. You might be able to get one from a system builder, but be prepared to pay the retail price for it.
Microsoft makes a TON of money off of their licensing, and they are very careful how they license systems.
They will not allow you to "mix & match."
Mark
Run Belarc Advisor (feerware) to see what apps and possible product IDs are on your machine. You may be able to use cd-keyreader to extract the MS OS product key.