InfoPath 2007 error when opening file

Question

We have some InfoPath (XSN) forms with code-behind hosted in MOSS that have data connections to SQL Server. I did some work on them in the summer to change one of the data connections - did the work, checked it into Sourcesafe, uploaded it to a UAT server.
We now need to do some more work on these forms, but no-one, including me, can now design them in InfoPath - they come up with:

InfoPath cannot open the following form: [Form Path]  
The file cannot be accessed.

Which isn't a very descriptive message. I'm hopeful somebody here might be able to suggest how to get these files too open again.

Edit: It seems that the copy of Microsoft.Sharepoint.dll inside the XSN file has become corrupted somehow.

Answer 1

The cause of this was the corrupted copy of Microsoft.Sharepoint.dll inside the XSN.