This behavior can occur if the Messages.ods file is not in the folder that is specified for it in the Windows registry.

To resolve this issue, use one of the following methods:
If You Do Not Want to Save Your E-Mail Messages
If you do not want to save your e-mail messages in Outlook Express 5, rename the Messages.ods file.

For information about how to rename a file, click Start, click Help, click the Index tab, type renaming, and then double-click the Renaming Files Help topic.
If You Want to Save Your E-Mail Messages
If you want to save your e-mail messages in Outlook Express 5:
1. Find out which folders contain the Messages.ods file and the .dbx files:
a. Click Start, point to Find, and then click Files or Folders.
b. In the Named box, type messages.ods *.dbx, and then click Find Now.
2. Move the Messages.ods file into the same folder that contains the .dbx files.

For information about how to move files, click Start, click Help, click the Index tab, type moving files, and then click Display.
3. Use Registry Editor to delete the ConvertedToDBX value in the registry:
a. Click Start, click Run, type Regedt32, and then click OK.
b. Double-click to expand the HKEY_CURRENT_USER folder, expand the Identities folder, and then expand the {clsid} folder.
c. Expand the Software folder, expand the Microsoft folder, and then expand the Outlook Express folder.
d. Expand the 5.0 folder, and then click the ConvertedToDBX value.
e. On the Edit menu, click Delete, and then click OK.
f. Quit Registry Editor.
4. Restart your computer, and then start Outlook Express.