If you have TFS Power Tools installed and Microsoft’s Communicator IM tool installed, you may get this error:  Team Foundation Error, Specified cast is not valid.

To resolve this error, open Visual Studio, in the Team Explorer portion of the IDE, right click on Team Members.  From the popup menu, select Personal Settings.  In the Personal Settings dialog, click on the Change button next to the Collaboration Provider choice.

And choose <None> and click the OK button, so now the Collaboration information looks like this.

Click the OK button on the Personal Settings dialog and exit Visual Studio.

Using your favorite TEXT editor, open your PersonalSettings.config file (C:\Documents and Settings\<user name>\Application Data\Microsoft\Team Foundation Server\PersonalSettings.config) and changed the xml element from

<CollaborationProvider connection="{none}" />

to

<CollaborationProvider connection="" />

and save the file.

Now open Visual Studio and it should launch with no errors.