Windows XP Default Mail Client

I am looking for a solution for setting the default e-mail client in Internet Explorer 6 SP1. Since I applied the service pack, Outlook Express is no longer listed under the programs tab on the Internet options panel and I am not able to manually insert it. As a result, whenever the client attempts respond to a web page that needs an e-mail client listed in Internet Explorer, an error occurs which causes XP to open 66 windows and crash. The error message states that the default mail client is not properly installed. Since only Outlook Express is installed on the workstation, the default should be there, but isn’t. Can anyone tell me how to resolve this?