Installing the same Windows license on several partitions

[Duce]

If it's XP, just grab the key via any app like Jellybean and install as you wish.

[Duce]

http://www.magicaljellybean.com/keyfinder/

Should work for pulling the key as a usable file to avoid a re-reg.

[Duce]

Jellybean and other keyfile pullers can pull the keyfile from the current install, allowing you to then copy that saved file to the new install and "activate" it, saving you the trouble of calling up MS to re-purpose/re-register the existing copy.

There's a number of them, but I always found Magic Jellybean to work the best, and it supports a load of apps.  Drop me a PM if it doesn't work for you and I can probably dig you up an alternative solution.