That's the renamed copy of your original inbox.dbx file. So the 2 steps (both elementary Windows use) are:
1. Restore from the recycle bin
2. Rename inbox.bat to inbox.dbx.
If there already is a new inbox.dbx delete that first (that's elementary use of Windows also). If it already contains newer messages you don't want to lose, move them to another folder before you delete it (that's done in Outlook Express).
Outlook Express running on my Windows XP asked me if I wanted to compress my files. I said YES and it apparently compressed a lot of folders by deleting them, including my IN Box. They are in my recycle bin as .bak files, but I cannot find the help I need on the internet to convert them back to readable emails. Thanks.