A constant problem with Outlook Express, it's the bane of users. Your .dbx files have become corrupted. There's some MS advice here...
The Other E-Mail Threat: File Corruption in Outlook Express
I downloaded DBXextract (now superseded by DBXexpress). Costs a few dollars/quid/yuan/brass washers, but it works.
DBXtract
I now run Windows 7 and use Mozilla Thunderbird, as MS no longer provides an inbuilt mail program.