I even did this from my old Windows 98 PC to my new XP PC.
Locate the folder from where you wish to retrieve your old emails: C:\Documents and Settings\username\Local Settings\Application Data\Identities\XXX
Copy the folder IDENTITIES and/or its contents, then save it to some removable media such as a CD-RW...then go to File>Import>Messages>Outlook Express 6 then browse/navigate to the location where you copied/saved your messages, and hey presto...after the import process has completed -all your emails & folders with your new ones.
Well it worked for me! Let me know how you get on :-)
Then Dig down into C:\windows\application data\Identities\FBEDAE0-11.......\Microsoft
and you will find a lot of files with .dbx extension.
Copy these to your backup.
If you need to restore them, just copy back into that directory.
However - there is a very handy little application called DBXtract, which allows you to read emails stored in the .dbx files from whereever they are. I use this regularly to read archived emails on CD and it has saved my bacon several times.
Perhaps someone else could direct you to DBExtraxt, as I have just realised that I have no idea how to insert a link into this message.