I ran into trouble once when trying to use something other than VSS--it did not exactly go gently into the good night.
If you ever need to check the registry to see what's up with your source code control software, the following folders are pertinent:
[tt]HKEY_LOCAL_MACHINE\Software\SourceCodeControlProvider\InstalledSCCProviders[/tt]
will have a list of the installed source code control providers, one of which (and probably the only one of which) should be
[tt][key]Microsoft Visual SourceSafe [data] Software\Microsoft\SourceSafe[/tt]
This simply means that SourceSafe is installed, and that (in this example, which is the default registry location) it's keeping information in the registry under
[tt]HKEY_LOCAL_MACHINE\Software\Microsoft\SourceSafe[/tt]
One folder up, in
[tt]HKEY_LOCAL_MACHINE\Software\SourceCodeControlProvider[/tt]
there will be the following key:
[tt][key]ProviderRegKey [data] Software\Microsoft\SourceSafe[/tt]
which will have whatever data value is in the installed source code control provider key I've listed above. This is the entry that tells what source code control provider is actually in use.
So, if VSS isn't in the list of installed providers, it isn't installed, and if it isn't in the ProviderRegKey key, it isn't the current source code control provider.
If this is all working fine, and you still have trouble, the problem could be in
[tt]HKEY_LOCAL_MACHINE\Software\Microsoft\VisualStudio\6.0\SolutionPersistence\SourceCodeControl[/tt]
This has some guids that the ide needs to interact with your sc provider. If this is off, you'll probably need to reinstall VSS.
HTH
Bob