The most reliable way is to search your computer for *.dbx files using Start –> Search. Make sure you check the option to search for hidden folders and files. Then backup any of the dbx files.
To restore these files on a new computer or new installation, Start Outlook Express, Click File –> Import –> Messages. Follow the wizard and choose Outlook Express 6 as the type of email you are importing, then click Browse to where you saved the original files.