When you are given admin on a server, your GUID is recorded in the server side document, admin.dat. When a player shows up trying to use an admin's name, but has a different GUID, the server will think that they are not the admin and therefore deny the use of the admin protected name. The only ways to use your name consistently is to keep a copy of your GUID when you re-install operating systems. Your GUID is saved in your qkey, which can be found in your tremulous directory. If you didn't save your qkey from before, ask a dev on US1 or any admin on EU1 and they can set your old GUID to level 0, and replace it with your new GUID. Remember to keep your qkey someplace safe.