It's probably giving you this error when you try to deactivate. Most likely, you're getting the error because you disabled one of your network adapters... for instance, if you activate the product on your wireless network adapter, and then you disable that adapter, the activation client will not be able to locate the MAC address for that adapter, and you will not be able to deactivate. If you are not getting this error when you try to deactivate, when are you getting it?
Also, if you have to force the removal of the license DLL, you can try to remove it from your C:\Windows\assembly folder. If the GAC will not let you remove it that way, you will need to open up a command prompt (cmd.exe) and change your directory to C:\Windows\Assembly\GAC_MSIL. Once you're there, you can "rmdir -s "MicroFour License"" (without the outside set of quotes). Then you should be able to reactivate.