Windows 7 Windows Mail / Thunderbird 3!!!!!!

Sorry if wrong forum.
On one of the desktop computers I am running Windows Vista with Windows Mail, on my new desktop computer (since last Friday) I am running Windows 7 with Thunderbird 3, I cannot export e-mail addresses from Windows Mail & import in to Thunderbird 3...................What's changed, your help most appreciated, because I've been messing about all day,