I have successfully exported Thunderbird email files on ms outlook 2007 on a Windows XP platform which can also be done in windows vista and windows 7 platform.  There are several steps on how to do it.

1.  First, you need to download and install this software:  IMAPSize
2. Locate your Thunderbird emails, usually it is located in this directory C:\Documents and Settings\user\Application Data\Thunderbird  or  you can copy paste the directory and change the user to your account name.  When you are in the application data folder of Thunderbird navigate to the mail folder of the email account you want to convert.  Make sure you make full backup on the mail folder before doing the conversion.
3.  Open IMAPSize software and go to the tools menu and select mbox2eml.
4.  Select the files you want to convert, one example is the inbox.  The inbox file that contains all you emails is the one without the .msf extension.  And this also applies for drafts, sent items and templates. Attachment will be automatically converted and goes with the .eml file

5.  After conversion, open Outlook Express on Vista or Win7 platform you can user Windows Live Mail.  Setup your email account on Outlook express.
6.  Highlight all the emails that was converted by IMAPSize which is in .eml format.  This should should store all the emails in your outlook express account.

6.  For the last part of the process, you can import your outlook express account on MS Outlook 2007.  On the file menu select Import and Export and click next, Select Import form another program or file and click next.  Select Outlook Express 4.x, 5.x, 6.x or windows mail.  This will automatically detect you windows outlook express email accounts and download all the mails.