Server 2008: The Installer has insufficient Privileges to modify this file
Sometimes the easiest things can be so difficult. I was just trying to install a simple update package, "Update Rollup 4 for Exchange Server 2007 Service Pack 1" to be exact. The installer returns the error "The Installer has insufficient privileges to modify this file: C:\Program Files\Microsoft\Exchange Server\RelNotes.htm".
If you've been using Vista for a bit, then you probably know about the good old "right click and choose Run as Adminsitrator" trick. Well, it doesn't work that way with MSI packages.
The way I got around this is by right clicking on a command prompt, and choosing run as administrator. Then changing directory to folder where I saved the MSI, and running it from the command prompt.