My Visual Studio 2003 debugger stopped working and nothing seemed to fix it. I tried uninstalling, reinstalling and fixing Visual Studio all to no avail.
For anyone else out there with this horrendous problem, the following eventually worked for me (courtesy of Microsoft Support):
1. Open a command prompt
2. cd /d "%CommonProgramFiles%\Microsoft Shared\vs7debug"
3. mdm.exe /unregserver
4. Open regedit.exe and check if HKEY_CLASSES_ROOT\CLSID\{73b25ffd-f501-437b-8b11-7f0de383964f} still exists. I think it will. If so, delete it.
5. mdm.exe /regserver
6. net stop mdm
7. net start mdm
8. Relaunch Visual Studio