Hi folks,
We recently had a strange issue with an InfoPath 2007 form being opened from SharePoint 2007 and receiving the error message "the system cannot find the file specified". To be clear, this was not a form services enabled form.
Anyway, after spending way too much time trying to figure out what was going on (nothing in the MOSS 2007 server log files), we determined that the user had previously installed an older version of the form (but with the same name) on their workstation using a no longer available msi file (meaning we could not uninstall it from the workstation).
So I wanted to pass on a very simple solution for anyone who is unfortunate to run into this problem in the future (since I lost a great deal of hair over it): Fire up regedit, go to HKEY_LOCALMACHINE->Software->Microsoft->Office->InfoPath->SolutionsCatalog. Locate the key that has the previously installed form name, and delete it.
This will cause InfoPath to stop trying to open the form locally (which is either old or doesn't exist) and force it to open your form from SharePoint.
Hope this helps someone!