Got this from click here
Why can't I set OE as the default email client? Why can't I Reply to Author in newsgroups? Why do I see a Choose Profile window when I Reply to Author in newsgroups?
All of these problems indicate that OE is not properly configured as the default email client on your computer. To be sure OE is the default mail client, close OE, click Start| Run and enter this command line, complete with quotes, spaces and the /reg switch:
"C:\Program Files\Outlook Express\msimn.exe" /reg
Of course change the path if you installed OE elsewhere.
Now click Start| Settings| Control Panel, double-click Internet Options, then click on the Programs tab, and verify that OE is now the default email client. (You can also get there from the IE Tools| Internet Options menu). Then open OE and click Tools| Options, and at the bottom of the General tab, verify that OE is set as the default. If it is, the "Make Default" button will be greyed out. If not, click the button.