Your system has forgotten which is your default email client.......you will need to re-associate it.
If you go ( this is XP which I think you have) control panel> add or remove programs. When that opens, in the main panel you will see three headers "Microsoft Windows", "Non Microsoft" and "Custom"
Put a check mark against Custom, then expand it via the little arrows on the right hand side. When that opens, run down to the email client section and check the box "use my current program" box.
Click apply/ok out of that.
That should do it.
edit to add the important bit