I guess you are using the SS.EXE (command line form of VSS) for doing the operation. If thisis the case then you have to register for the same by modifying the Environmental variable found in
Start>>Settings>>Control Panel>>System and press the "Environment Variables" button on the "Advanced" tab. Find "Path" and append ";[path of win32]" to it.
SS.EXE is located in the "win32"directory under the VSS installation and this "win32" directory must be in your path.
Guru
Regards
Guru (CLA)