According to: http://support.microsoft.com/?id=301474
the JET 4 service packs address this issue. Use the Windows Explorer to
locate the file msjet40.dll (typically in windows\system32). Right-click and
choose Properties. On the version tab, should should see:
4.0.8xxx.0
where xxx can be anything. But if you don't see the 8, you need to download
the latest JET service pack from: http://support.microsoft.com/kb/239114
Once that's correct, try compacting the database:
Tools | Database Utilities | Compact/repair