Choli, I did the initial installation of MP3! being the admin. As far as I can tell the 'C:\Program Files\Messenger Plus! 3\' folder is accessible to the Guest (also, I did not make any changes to the access rights anyway after the installation and before MP3! it simply was not there).
However, I (logged in as the Guest that is) have just now tried to create a txt-file in that folder and for some reason XP locks me out. No access rights... I have checked under the Admin, but as far as I can tell there, no access restrictions are in place for this folder, nor its content. It's also not hidden for the Guest. Strange