Hmmmm,

I wonder if something has happened to the Windows installer?

You might try this:

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