Autocompletion of addresses in Windows Mail does not use the contacts
list. Instead, it uses a separate list of the last 29 addresses you sent
messages to.
To turn on this autocompletion, click on Tools, then Options, then Send.
If there is no checkmark in the box before Automatically complete e-mail
addresses when composing, click on this box, then Apply. Click on OK.
If you'd rather use the contacts list, start a new message as usual, except
don't type any part of the address. Instead, click on To:.