When using Visual Studio 2005 or Team Explorer to connect to your Team Foundation Server for the first time, you get to option to provide alternate credentials.

After doing this, there is no clear way to reset those credentials. There are two solutions:

a) Run 'ClearCreds.exe' which is part of the Visual Studio 2005 SDK.

b) Go to 'Start|Run' and enter 'control userpasswords', next go tabpage advanced and choose  manage passwords, now remove the server which is your Team Foundation Server.