Go To Tools -> Options -> Maintenance -> Store Folder.
This should give you quite a long directory name. EG.
C:\WINNT\hgghgg\Application Data\Identities\{F44E0000-G82F-89F5-2647-00605AD72F7J}\Microsoft\Outlook Express
If you navigate to the folder just copy all the files in that directory to where you want to save them to. Then when you re-install everything Create a folder on the C: drive such as emails. Copy the files to this folder. Then go back to the store folder option and change the store folder to C:\emails (or whatever you called your new folder and put the files into).
The next time outlook is started all your e-mails will be there.
By the way, Don't forget to backup your contacts list as well. The best way for this is to use a text file CSV (comma separated value). Go to File -> Export -> Address book. text file CSV -> export. Then save the file wherever you want to. To import it just go to File -> import -> Other address book -> text file CSV -> Import-> Browse. Dont forget to change the files of type to *.txt.
Anyway hope that helps!
By the way dont forget you can also save your e-mail account settings (such as pop3, Username & pass), by going to Tools -> Accounts and using the import and export buttons there.