Hi swerdna. Which email client are you looking to use in Windows 7?
I realise this doesn’t actually answer your question, but I’m thinking only about extracting your valuable mbox files. Forgive me if I’m off track, and you may know this process already… but if using Outlook, there is a free Windows app called IMAPSize with a utility called mbox2eml that can convert mbox email files into eml files (digestable by outlook). Essentially you would need to do this for every folder you have set up in Thunderbird. (That may be a tedious process for you depending on your email folder organisation).
How to Export Mail from Thunderbird to Outlook Express and Other Email Programs - About Email