You can't just copy the accounts and messages. Here's what to do. First make sure that you are running the same version of Outlook Express on both machines. (Ver 6 is the latest).
On the old machine you will have to open Outlook Express, Click on file and the import/export option. Here you can export your contacts to a file. Put the file in a location you can easily get to on the new computer. Next click on tools and accounts. You will be able to export your accounts in a similar way. On the new machine repeat above and select the import options and import your saved files. That's all there is to it.