Don't worry. It basically means, that the particular Serial is not not being used.
If you don't revoke the key , and try to install the game to different computer using the same key, it will fail.
Because the key is already in use. To be able to pay on the second computer, you will have to revoke the key from this one. This is the DRM thingy, which restricts one DVD / system .
So go ahead and revoke your key, and uninstall. The key can be used later.
I have done it multiple times with my copy.