I had a bad install of Visual Studio 2015 Enterprise (iso from MSDN Subscription) and it would not run or uninstall, even with /uninstall /force.
Some of the error messages:
"Failed to download the update xml file..."
This tool did the trick, it tells you where all of the .msi files are so you can uninstall Visual Studio bit-by-bit. All of the uninstalls worked. I probably missed a few, but when then I did the web install and selected uninstall and it uninstalled cleanly. I then was able to do a full web install that went perfectly.
Thanks to Aaron Stebner.