Thanks for your help everyone. I managed to solve the issue.
The installation errors for *.svn we're solved through a simple %PATH% change.
For posterity, the culprit behind the main topic error was how Windows 7 ran Python, which prevented arguments to be run by Python executable. This is a known issue, with more details available here:
bugs.python.org/issue7936
I identified this when I recieved the error "errno2: no such file or directory" when I typed in: "C:\opentelemac-mascaret\python27 telemac3d.py"
A simple "assoc .py=Python.File" didn't work.
I ended up changing the registry keys for HKEY_CLASSES_ROOT\Applications\python26.exe\shell\open\command and HKEY_CLASSES_ROOT\py_auto_file\shell\open\command to "C:\Python26\python26.exe" "%1" %*
Previously the %* had been missing from both.