It's a known issue. See the announcement at the top of the forum.
This problem is because of a Microsoft security update that has been released recently!
Please go to control panel/Programs & features
here you will find "View Installed Updates" at the top left hand side corner.
click on that & search for KB2803821.
Uninstall the update & reboot the computer.
It should start working.
Thanks for posting the actual solution, avanishdivya.
Gah! Sorry I didn't see the announcement. Thank you so much.
It appears that Microsoft has issued the fixed update:
See the "Update FAQ" section:
"Why was this bulletin revised on August 13, 2013?
Microsoft revised this bulletin to rerelease the 2803821 update for Windows 7 and Windows Server 2008 R2.
The rereleased update addresses an application compatibility issue in which WMV encoded video could fail to properly render during playback. Windows 7 and Windows Server 2008 R2 customers should install the rereleased update."