All contacts you ever created in Outlook Express are stored inside the address book, in an internal format, preferred by this email program; but the information itself is plain text, for the most part, and can be exported to a variety of file format. This feature is particularly useful if you want to migrate your address book to another copy of Outlook Express (i.e., on another computer), or if you want to export your contacts and import them into another email program, like Windows Mail or Windows Live Mail. But let's start with a related tip:
Tip: depending on what you want to do by exporting your address book contacts, here is an alternate approach, quicker and easier, which could help you. From the main Outlook Express window, click on the Tools menu, and choose "Address Book". When the address book window opens, click on its Edit menu, and choose "Select All" - now go back to the Edit menu, and choose "Copy". Outlook Express has copied to the Windows clipboard all your contacts, which can now be pasted inside a text editor of your choice, like Microsoft Word or Notepad!
Follow these simple steps to export your Outlook Express contacts to a file:

WAB, which is the most common need, and the one you typically use to switch to another Microsoft email program. Note that given Outlook Express's popularity, several third-party email clients now support the exported WAB format for address books.And you are done! Depending on the speed of your computer and the size of the Outlook Express address book whose contacts you want to export, the process may take a few seconds. Outlook Express will display a confirmation message letting you know when it successfully finished exporting your contacts: "Your address book has been successfully exported to [folder path] [file name] . WAB".
Click OK to close this confirmation message; you can also close the address book and return to Outlook Express; your contacts have now been backed up, if this is what you wanted to achieve; otherwise, they are ready to be moved to another computer and/or imported into another email program: see how to import contacts in Outlook Express.