First confirm that you can create a new project from scratch.

Do you have more than one provider, perhaps clearcase? If so there might be
a problem with the SCC registry values.
Have a look at HKLM\Software\SourceControlProvider. If you have more than
SourceSafe listed, export those values and remove them. ( except for vss
)Try your open. If the issue is resolved, have a look at the following on
how to re-integrate the other providers:

820833 PRB: ("Unspecified Error") Error Message When You Perform a Source
Control
http://support.microsoft.com/?id=820833

If this is a web project, you might be running into another issue where the
.tmp file extension has been added as a MIME type in IIS. Start IIS manager
and add that MIME type. Try your open from Source Control.

Lastly, try running ssint.exe from the VSS\Win32 directory. This executable
re-writes the reg keys for integration.

If all of this fails, I would encourage you to give us a call for more in
depth trouble shooting.

Chris Cooper
Microsoft

This posting is provided "AS IS" with no warranties, and confers no rights.