One of the things which will occur for certain developers is a need to be able to connect to both TFS and VSS from the same development PC.  This may occur as a company transitions from VSS to TFS or for consultants when you move from working with company A who is using TFS to company B who is not.

In my case I went from having been configured for TFS to needing to install VSS for a client.  Once I installed VSS, my ability to connect to TFS source control seemed to disappear. 

Both the VSS and Team Explorer clients can be installed on you machine.  However, you have to make a choice between which server side source control data store you will use.  To do this from within Visual Studio go to the Tools menu and select "Options..."  In the Options window select "Source Control"  from the list of areas for customization and you can go from there, as shown in the screen shot below: