type "features" into the start menu search, choose "turn Windows features on or off"
see if IE8 is there, if yes, uncheck it, reboot. after that try installing IE9 again. if it doesn't work (and/or you want/need IE8 back, repeat the procedure, this time checking IE8 and reboot)
JUST IN CASE, I would download an alternative browser, if it so happens that you get stuck at some point and don't have any browser to actually get more help online. PortableApps have Chrome and Firefox portable, just install it on the desktop or something, and use it in case you need it, if not, you can just delete the folder