How to debug "Class not registered" errors on Windows
I spent a few hours installing Visual Studio 2008 today, but got into a weird situation when one of the install components hung. When I killed it, I left my system in a weird state, and after a bunch of reinstalling, I could get VS2008 to start, but I couldn't create an outlook 2007 add-in project (other projects created fine). I kept getting a "Class Not Registered" dialog that told me nothing about what was being looked for.
Turns out the way to debug this is to use RegMon. Look for OpenKey operations by your offending app that result in NOT FOUND responses. Of these, look for those that access the CLSID part of the registry, and then note the GUID's for the classes. You can use RegMon to copy each entry. Paste it into notepad and pull out the GUID and google it. Usually you can find something about what component it is associated to. Reinstalling that component usually fixes the problem.
In my case, it appears that when I uninstalled some SQL server component, It also took out some MSXML core services dll. After I reinstalled this, everything works smoothly.