I've had these two problems plaguing me for a long time, and they both started at the same time so I'm assuming that they're related.
When I try to install a program that uses MSI packages, I get an error saying that the MS Windows Installer Service failed to start. I can install anything else (Install Shield stuff, Nullsoft Installer, etc) but not MSIs.
Also, when I get into certain programs (MS Office, SolidWorks, etc) I get a different error regarding document registry. In office, I can open a document but it says something similar to "This document could not be registered, It may not be possible to create links from this document." In SolidWorks, I can't open a document that's already been saved because it says "Unable to register document, it may already be open."
What's going on is beyond me, but it's interfering with my college work and work work! I give up, someone help me!