You have got a few options, you could just do an upgrade of the OS, that will leave everything intact.
Ya could do this (taken from help files in outlook)
To compact and back up e-mail folders
Select an e-mail folder.
On the File menu, point to Folder, and then click Compact.
To back up the folder, find the folder file, and then copy it to a backup folder, floppy disk, or network drive. Outlook Express e-mail folders have a .dbx extension (for example, "Sent Items.dbx").