Reset Default Mail Client In Windows XP

To reset the default email client on Window Browser, go to the Start Menu, choose Control Panel and Internet Options.

OR Go to Internet Explorer, Tools > Options, and selecting the Programs tab.

OR One more & common option :  Go to Internet Browser Properties from desktop there are all Internet program including email applications listed on the program tabs of the Internet Properties window/dialog.

Then Click Programs tab and select your email client from the drop-down list. Once selected, click OK/Apply button.