I am trying to export my messages from Outlook

To resolve this issue, try one of the following methods, as appropriate for your situation.

Method 1: Reinstall Outlook temporarily
If you are importing mail from Outlook into Outlook Express, but you have already removed Outlook from your computer, reinstall Outlook, and then try to import the mail again
Method 2: Change your MAPI Settings
Configure Outlook Express to not be the default Simple MAPI client before you import the mail items. To do this, follow these steps:

1. Restart your computer, and then start Outlook Express.
2. On the Tools menu, click Options.
3. On the Connection tab, click Change to open the Internet Properties dialog box.
4. On the Programs tab, click an e-mail application other than Outlook Express in the E-mail list. Click Apply, and then click OK.
5. Click OK again to close the Options dialog box.
6. Exit Outlook Express, and then restart Outlook Express.
   
   You can now import or export messages.

NOTE: After you complete the import or export process, you can configure Outlook Express to be the default MAPI client. For more information about how to configure Outlook Express to be the default MAPI client, click the following article number to view the article in the Microsoft Knowledge Base:
Set Outlook Express as your default Simple MAPI client Or u can refer from here: http://support.microsoft.com/kb/179637

I dunno but you might be better off just copying the outlook express data files from one computer to the next instead of trying to export.
http://support.microsoft.com/kb/270670
from the horses mouth.