Using Win98SE?
First back up your Address Book to a Text File (Comma Separated Values) file.
File | Export | Other Address Book | Text File (Comma Separated Values) 
Give it a name and the   .csv    extension.
Then finish the export, it will ask which fields to export, check them all.

Now, remove OE.
Control Panel | Add\Remove Programs | Microsoft Outlook Express
Press the Add\Remove button.
OE will be removed.
Next, open Windows Explorer, go into Program Files\Common Files\System and
rename the DirectDB.dll file to DirectDB.old
Shut down and reboot.

Upon restarting, Control Panel | Add\Remove Programs
Select the Windows Setup tabsheet.
Select Outlook Express 5

If you don't have Win98SE, you may run into a problem of getting an OE5 only setup with IE and may have to uninstall IE5 as well. You might look around and find IE5 on a CD, lots of them around with it on there.

Control Panel | Add\Remove Programs | Microsoft Internet Explorer 5.5 and Tools
Press the Add\Remove button
Press the Advanced button
Select to restore OE as well. It won't but it does remove it.

Once done, reboot and install IE 5.0