Inbox.dbx is the inbox file for Outlook Express.
Outlook uses 'pst' files, rather than 'dbx' ones.
Thunderbird doesn't use of those file extensions; it has a completely separate inbox.
If you can't download e-mails with either Outlook or Thunderbird, it's likely that the problem is external to your e-mail clients. (e.g. it could be associated with your firewall or your port settings). However, if you still want to try another e-mail client, you could install the Opera web browser, which incorporates its own e-mail client:
http://www.opera.com/products/desktop/m2/
Chris