A small troubleshooting and how to fix it.
Problem: Internet Explorer 7 under Windows Vista doesn't plays flash elements (such as Youtube or Soapbox videos). You install the Adobe Flash Player 9 component, restart and still don't works.
Answer: Download a stand-alone installer, such as the one from www.filehippo.com, and install it. Restart IE7 and you should now have flash player activated/available for all webpages.
Update: Thanks to the fallen one, in this thread there's another workaround to fix this without reinstalling Flash.