I had an interesting issue where somewhere in the past I had used the guest account inside Visual Studio to connect to Visual Source Safe. However, since then, Visual Studio remembered this login ID.
It is visible using Tools->Options->Source Control->SCC Provider and it showed "guest" and was disabled.
Today I wanted to access this database using my own account, only Visual Studio stubbornly kept using the guest id.
I already had used the SSUSER environment variable to set the correct user; but although this makes the VSS explorer use the correct user, in this case it did not help to select the correct user for VS 2003.
In the end I performed this trick to make it work:
1. Temporarely rename the "wrong" ID (in this case 'guest') to something else.
2. Restart Visual Studio and open te source controlled project
3. Visual Studio will ask for an ID and Password because the one it knows does not work
4. Fill in the correct ID and password.
5. Close Visual Studio
6. Rename the "wrong" id back to its orgininal name.
This is the way it has worked for me; although I'd rather see a better solution for this problem....