Here's a suggestion. Don't know if it will work, but it may.

1. Use any other key you can get your hands on (I'll leave that up to you) to perform the install - this is only temporary, just to get you to the desktop.

2. Use a key changer tool to put the original CD key back into the registry, replacing the temporary one used above.

3. Activate.

Obviously you can force #3 with yet another tool if it doesn't work automatically/properly.