Visual Studio and Team Explorer provide a caching mechanism which can get out of sync. If I have multiple instances of a single TFS which can be connected to from a single Visual Studio client, that client can become confused.
To solve it..
For Windows Vista delete contents of this folder
C:\Users\{your account}\AppData\Local\Microsoft\Team Foundation\1.0\Cache
For Windows Xp, 2003 delete contents of this folder
C:\Documents and Settings\{your account}\Local Settings\Application Data\Microsoft\Team Foundation\1.0\Cache
I'm still getting the E_FAIL from a COM Component error.