This video is no longer availableeven though you know that the video works with Firefox, Opera, or other browsers.
In that case, it is a proxy issue. You are trying to download the video with a proxy that is incompatible with YouTube. A very typical example is Google Web Accelerator, a program that should speed up the internet traffic by using a Google proxy. If you use it, the fix is simple: right-click the accelerator icon in the tray and stop the accelerator.
If you still haven't fixed it,
- try to download hotspot shield that gives you a new proxy IP address to download from the U.S. YouTube (most complicated)
- or try to change/increase the quality by attaching a "fmt" parameter to the URL, e.g. http://www.youtube.com/watch?v=WQyMnoYqguo&fmt=6 - here, the numbers 6,18,16 at the end can give you nontrivial results. A new server may be chosen.
- choose your own proxy. In Menu / Tools / Internet options / Connections / LAN settings, check the "use proxy" switch and enter 188.8.131.52 as "proxy" and 8080 as "port". Answer the dialog boxes and try to view the videos.