From http://support.microsoft.com/?kbid=258034

Ensure that your .DBX files are not Read Only:
Your .DBX files are the store files for your Outlook Express folders. If your files are marked as Read Only, they cannot be accessed. An example of a cause of this would be if you recently restored the files from a backup on a CD-ROM. To remove the Read Only attribute, perform the following steps:

• Click Start, click Find (or Search) and then click Files and Folders.
  
• In the Name field, type *.dbx, and then set the search to include local hard drives.
  
• Click Find Now or Search Now, and when the results are displayed, click the In Folder column heading to group items by location. Note the locations of the files found and then use Windows Explorer to navigate to the directory that contains your current store.
  
• Right click the first file in the folder, click Properties, on the General tab, click to uncheck the Read Only check box, and then click OK.
  
• Repeat step 4 for each file in the folder and test Outlook Express.

There's also another fix indicated, but it requires updating the registry.

Good luck Rick