A client was having a problem viewing videos on YouTube using Internet Explorer (IE). He has Vista installed on his laptop. I had spent some time trying to resolve the problem and when I could not figure it out, I suggested that he install Firefox. He was able to view content using Firefox.
I continued to research the problem viewing videos with Internet Explorer (IE) on a Vista workstation. I then finally came along a post at https://forums.techguy.org/ that explained the problem and provided the solution. It seems that even though the Flash installer says that the new version is installed, it requires some manual intervention.
Go to: C:\Windows\System32\Macromed\Flash\ and double click on the file FlashUtil9b.exe. This will execute the software update that makes it available to the Internet Explorer (IE) browser. Reboot your system, navigate to the Youtube video you wanted to play and all should run as expected.