To undo any settings that you have mistakingly made try using System Restore, or right-click on the file you want your E-mail program to open and select "Open with", and then the Browse option to find the appropriate E-mail program.
How to repair the operating system and how to restore the operating system configuration to an earlier point in time in Windows Vista
http://support.microsoft.com/kb/936212/#appliesto
Do you have your E-mail program listed here?
Control Panel\All Control Panel Items\Default Programs\Set Default Programs. If so make sure it has all the defaults set.
If not see this article.
Change default email program
http://www.webdevelopersnotes.com/how-do-i/change-default-email-program.php
Change the default e‑mail program
http://windows.microsoft.com/en-us/windows-vista/Change-the-default-e-mail-program