To make Outlook Express your default e-mail client:
Open Outlook Express
Go to Tools > Options > General
At the bottom of that section, you'll see 'Default Messaging Programs'. Alongside the 'News Handler' entry, click 'Make Default'
To change the default e-mail address you send messages from:
Go to Tools > Accounts > Mail
Click on the account you want to be the default account.
Click 'Set as Default'.
it won't let me do it, the problem is that it keeps saying as the mail server address, btopenworld whereas it is mail.btinternet.com. For some unknown reason, whatever you do, you cannot change the original adresss.