If you have uninstalled & reinstalled your copy multiple times and now subsequently used up your max number of activations you can sometimes ring the Activation line at Microsoft and explain to them what has happened and they can allow you to activate it over the phone and they will give you the code to manually enter.
You just need to make sure you have not exceeded your software licence by installing it on more computers than you should, and it should be uninstalled from the old computers where possible.