It's probably best if you login to your Nominet online sevices account and delete the domain, after logging in, click Your Domains on the top menu bar and then click on Cancel Domains.
If you set up a forum on it, you would not be able to put any Adsense or affiliate ads on there, if they showed products for Microsoft or their competitor, they would have you bang to rights, so it's not worth starting a forum for which you will be paying the upkeep of, in return for no revenue from it, easier to delete it, put it down to experience and stay away from TM names.