I recently almost hosed my windows 7 installation, when I tried to install Internet Explorer 7 instead of Internet Explorer 9. The reason I tried to do that, was that I needed to have access to a really ancient corporate site, that looked hopelessly scrambled in IE9.
What I hadn’t realised was this was a recipe for disaster; IE7 is incompatible with Windows 7!! After IE7‘s installation failed (obviously), I was left with an impossible situation: I could not start any program and I could not even open up a folder with windows explorer. Instead, I would get a pop-up window with title «Explorer.exe», and the very friendly message «No such interface supported«.