The bindings are propbably bad - when trying to use the old solution.  Verify
that the local working directory is set correctly while using the new VSS.  
Start fresh by getting the solution from VSS using the VS open from source
control command (File -> Source Control -> Open From Source Control...).  
This is also a good test that the code you need is in VSS.  

You might also have to check and fix the bindings.  The binding can be
viewed and changed using the "Change Source Control..." command (File ->
Source Control -> Change Source Control...)

If you are going to use both the old and new VSS, make sure that different
local path is used for each.  The bindings in each solution are different.  I
expect you can just fix the bindings in the old solution if you will not use
the old VSS.  Best of luck.