Windows Installer Service issue - Vista 64bit

[Flogger23m]

It seems like I have a Windows Installer Service issue. My OS is Vista 64bit Home Premium.

Certain programs are not installing (Windows 7 Upgrade Advisor to name one). Win 7 Upgrade Advisor gave this error message when trying to install:

The Windows Installer Service could not be accessed. This can occur if the Windows Installer is not correctly installed. Contact your support personnel for assistance.

Most programs install fine, but this, .NET Framework 2.0, and some other installers have issues and can never install. I have not had any issues uninstalling programs.

What can I do to repair the Windows Installer Service or fix this issue? Thanks in advance for any help.