If you can't view videos on Entensity or other sites one of these SHOULD work.

1) You could uninstall/reinstall windows media player...
2) Locate the following plugin files in the windows media player directory, and copy them into the firefox plugins directory (links provided in case you can't find or want to dl them and just put them into the firefox folder):

npdsplay.dll Download here.
npwmsdrm.dll Download here.
npdrmv2.dll Download here.
3) Go to, Download here. and download the file (even though it says for netscape navigator). run the downloaded file...and all should be fixed.
4) This may help for VISTA users: Download here
5) If above solutions don't work, remove any third party audio/video plug ins and try to figure out which one is interfering. Also I've found media encoders such as SUPER and MediaCoder interfer.
6) Necessary codecs that may be required:
Windows Audio 9.1
Windows Media 9 codecs
Windows Media 9 VCM codec
FFDSHOW - You should have this anyway.
7) You should also make your firewall/antivirus isn't causing problems as well AND use IE6+ or Firefox.
8) For flash videos download this

UPDATE: ANDROID USERS JUST CLICK THE VIDEO WHILE IT BUFFERS AND IT WILL PLAY