To change back to VSS from Clearcase:
In the registry there is are two keys
1) HKEY_LOCAL_MACHINE\SOFTWARE\SourceCodeControlProvider
and
2) HKEY_LOCAL_MACHINE\SOFTWARE\SourceCodeControlProvider\Inst alledSCCProviders
In the key #2 you should see two entries:
ClearCase=Software\Atria\ClearCase and Microsoft Visual SourceSafe=Software\Microsoft\SourceSafe
What you need to do is delete the "ClearCase" value, so that only the "Microsoft Visual SourceSafe" is left.
In key #1, make sure that the ProviderRegKey is set to "Software\Microsoft\Visual SourceSafe".
Alternatively, just register the ssscc.dll in your C:\Program Files\Microsoft Visual Studio\VSS\win32 with the regsvr32.exe tool and the Default Source Control Provider will change back to VSS.
And thats it!