Before you sell or recycle an old computer, you might want to take your Windows licence and use it on a different PC. The How-To Geek offers instructions for uninstalling your product key and installing it on a new PC.
You just need to open the Command Prompt in administrative mode and run two commands to uninstall the product key. Installing it is just as easy using the same "slmrg" command.
You should note this fine print from the article before proceeding:
Note: Between Microsoft and your OEM there are a number of different laws that determine whether you are allowed to use your licence on another PC, so follow this tutorial at your own risk. Additionally before you go through the tutorial you will want to check that your product key is still attached to your PC/Installation CD, if it is not, do a quick Google search on how to recover it.
For the full instructions, see the tutorial below.