One way:

Open internet explorer
Tools|internet options|Programs tab
change your email client here.

This can happen if you download spam filter updates to Outlook.

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For what it's worth, after I download those filter updates, the same thing
happens to me. But then the next time I open OutlookExpress, I get a prompt
asking me if I want to use OE as the default email client.

Jill Hatt wrote:

For some reason when i email from excel my puter has started using Microsolft
Outlook and I want it to use Outlook Express (which it did originally) , how
do I change this default?

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Dave Peterson