Three separate methods! First, a set of .REG files to merge; you can use these to fix only what is broken, if you know:
http://www.sevenforums.com/tutorials/19449-default-file-type-associations-restore.html
Second, you can delete a particular area of the registry, and it will be re-created at reboot:
http://www.windowsvalley.com/restore-default-file-association-in-windows-vista-and-windows-7/
And third, a utility:
http://www.thewindowsclub.com/file-association-fixer-for-windows-7-vista-released