I was looking for a solution for the exact same problem when I found this thread. I have managed to resolve the problem though. Here's how my trouble started and finished.
A system was brought to me with "problems" and I was given little detail. I found that when Internet Explorer or any Office application was opened, the Windows Installer started. At first, I had to put the XP OS CD in to get IE or the Office app to finish opening. I did find out that the user of my system had uninstalled Office XP Standard and installed Office XP Pro and that's when this issue started. I reinstalled Office XP Pro and the trouble went away. I think the installer somehow got "stuck" and when I did the reinstall, what ever file that was currupted, got fixed.
I hope you can take from what I did to help you resolve your problem.
Matt