Turning AutoComplete Addresses On or Off
 

I found this in a few different places around the 'net - hopefully it's helpful:

Turning AutoComplete Addresses On or Off

The AutoComplete feature in Outlook Express in Windows XP saves you time by automatically completing addresses that you type when composing e-mail messages. However, if you don't want to use this feature, you can easily turn it off.

• On the Tools menu in Outlook Express, click Options > Send tab, and then clear the Automatically complete e-mail addresses when composing check box.