Could your Guest account be one that you created and named it "Guest". If so then this is not the proper "Windows" Guest account. It will not have the restrictions that the Windows Guest account has and that may explain how it was able to be changed to an Administrator account. If this is the case then try changing the "Guest" account name to something else (Win25?) and enable the correct Windows Guest account. From the Win25 account try changing your account to an Admin account. If it works you can then, from your account (which is now Admin) you can get rid of the Win25 account.