Outlook Express as default email program

I just installed XP Home SP2 on a new computer. I also installed Office XP.
I am not able to set Outlook Express to be the default email program. If I reset the program to be the default email program in OE/Tools and go to Internet Options/Programs, the choice that comes up will be blank, with Outlook being an additional choice.
If I click on a mailto link in a website, I get a message that the mail program is not installed correctly.
I successfully imported all the old email and address book to OE, and it seems to be working properly.
What could the matter be? The computer is clean of viruses and spyware.