Vista - Exporting Windows Mail Message Problem

We use Windows Vista Home Basic, Service Pack 1, 32bit. We have had intermittent sucess in exporting Windows Mail Messages. We perform the traditional steps and receive an error.

Steps:

1. Open Windows Mail.
2. Click File.
3. Point at Export and then select Messages.
4. Select Microsoft Windows Mail in the Select Program window and then click Next.
5. Click Browse in the next window to choose a place to save the backup.
6. Navigate to where you want to save the backed up e-mail in the Save box.
7. When you’ve navigated to where you want to save the backup, type the name of the backup in the Folder box and then click Select Folder.
8. Click Next in the Windows Mail Export window to select what to export.
9. Select All to export everything or choose a certain folder to export messages from in the Select Folders window.
10. Click Next to start the backup.

We receive below error messages:
"An Error has occured" -- Windows Mail was unable to export all your messages.
Your disk may not have enough space for all the exported mesages, or some of the messages you're exporting may be corrupted. Run Disk clean up or delete some files then try again.

We have 91 gig of space available. Our last mail export was 80 meg. We have performed disk clean up and still get same message. What are we doing wrong?