Which version of Internet explorer are you using ?
You might see if uninstalling flash helps, Flash does not update well I've found,
http://kb2.adobe.com/cps/141/tn_14157.html
Silverlight/ only Microsoft uses it and I suppose Netflix to stream,
Post a link to a video your unable view,
Sorry if some of this has already been suggested,
If using ie11,
http://www.youtube.com/html5
Internet explorer,
Make sure Adobe shockwave flash element is Enabled, Use Alt+TA or Gear symbol on the top right and Manage add-ons.
Switch to Software rendering,
http://www.sevenforums.com/tutorials/149063-internet-explorer-gpu-hardware-acceleration-turn-off.html
Use the (Do I have Java) listing on this page,
java.com: Java + You
See this for flash player troubleshooting,
Flash Player Help | Flash Player Help
Check your Silverlight version here,
http://www.microsoft.com/getsilverlight/Get-Started/Install/Default.aspx
If you see a Blue circle with a slash through it,
Right click it to Disable Active x filtering,
Also available with Alt+T or Gear symbol on the top right and Safety/ Uncheck Active x filtering.