Unfortunately the Contact List has been removed in Windows Mail. You could put the Contacts in the Sidebar, via the Windows Orb (Start) > All Programs > Accessories > Windows Sidebar then add the Contacts (and any other gadgets you want) by clicking the plus sign at the top. Now click on the Contacts ‘shaded’ box and drag it to the Desktop to give a bigger and better display as an ‘open book‘. To compose an email, scroll down the left page and click on the Contact you want then click on the email address in the right page to open the New Message dialogue.
Windows Mail auto-completion doesn't use the Contacts list (address book). Instead, it uses a separate list of the last 29 addresses you sent messages to. That list is stored in the registry and is not easy to understand or edit. Names can be partially recognized by right-clicking the identifier (a-z, etc.) then clicking Modify. They are stored in the registry key HKEY_CURRENT_USER\Software\Microsoft\Windows Mail\Recently Used Addresses.
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