We all have been easily able to do this prior to Windows 8 (in XP/VISTA/WIN7) with few changes in Registry and it was done.
With Windows 8 it\'s not that easy (I\'m
For all those waiting for answer to this question.
Here you go, looks like that one of the way to do it is through DISM.exe provided by Win 8. Your user should be admin of the machine though.
Using C# code you should be able to run this DISM.exe with specific parameters.
Following blog has the details.
http://blogs.technet.com/b/mrmlcgn/archive/2013/02/26/windows-8-associate-a-file-type-or-protocol-with-a-specific-app-using-a-gpo-e-g-default-mail-client-for-mailto-protocol.aspx
Thanks.