From Aaron Stebner, setup guru.
Aside: Aaron is one of the unspoken heroes for Express. For those of you who don't know, if you don't uninstall Express in the correct order, bad things happen. To help make this easier, Aaron had a tool to do this for you (which was later changed and cleaned up by the setup team). I think this is awesome primarily because this isn't even his job (he didn't work for DevDiv at the time), he just went above and beyond and to this day, he has the best most detailed instructions on troubleshooting setup.
Bonus post: Ever so handy: Command line switches for Windows hotfix setup packages