Due to some problems with my Visual Studio 2005 installation, I had to reinstall VSTS on my PC. This caused my VSTO Outlook projects to stop working in VSTS with the error message "Unable to load project file <file name>". No further details were given. I then reinstalled VSTO-O beta 2, the VSTO beta 2 run-time (VSTOR.EXE), and the Office 2003 PIAs. Still no luck. I then reinstalled the Windows Script 5.6 as this had solved a VS.NET problem for me before. My project would still not load.
I then tried to create a new VSTO-O project, and this time I got the famous "class not registered" error. As usual, no details about which class.
It was time for RegMon from System Internals, a company that provides high quality freeware tools for "under the hood" monitoring during deployment and troubleshooting. I opened VSTS, selected 'File-New-Project' and browsed to the "Outlook Add-in" project template. Then I captured all registry access done while I clicked OK in the VSTS 'New Project' dialog. RegMon captured a looong list of trace, but by filtering on process name "devenv*" the list was shortened to only a few hundred entries.
Starting from the bottom of the list, I soon found two "OpenKey" events that resulted in "NOT FOUND", with no further attempts to read info about the CLSID. Double-clicking the entry in RegMon opened the standard Registry Editor, in which I myself could see that the specified CLSID was nowhere to be found in the registry.
I then used another PC with a working copy of VSTS and used the standard Registry Editor to search for the CLSID (GUID), and it turned out to be MSXML6.DLL that was missing on my PC. You will find this DLL in the System32 folder.
I think that all this was caused by uninstalling SQL Express from my PC. It would not be the first time Microsoft installers do not mark shared DLLs as "permanent".