A common troubleshooting Windows Installer technique before vista was to reregister the Windows Installer service.  The steps went something like this

Unregister Windows Installer, and then reregister Windows Installer. To do this, follow these steps:
 
1. On the "Start" menu, click "Run:.
 
2. In the "Open" box, type "msiexec /unreg", and then press ENTER.
 
3. On the "Start" menu, click "Run".
 
4. In the "Open" box, type "msiexec /regserver", and then press ENTER.

This no longer works on Vista and above.  This is because the Windows OS install technology itself, called CBS aka Component-Based Servicing, has moved all service registration into it's own declarative format.  CBS also uses an underlying technology called service hardening to have it's Trusted Installer service own all of the OS resources. Using servicing hardening enabled Windows Vista to evolve Windows File Protection into Windows Resource Protection as it now protects both files and registry keys.

References to this troubleshooting step for earlier versions of Windows include