The outlook express email in Windows XP is usually in
C:\Documents and Settings\<your loginid>\Local Settings\Application Data\Identities\{some lengthy alpha-numeric serial ID}\Microsoft
The "Local Settings" directory is marked hidden, so you'll have to have "view hidden files and directories" enabled to see it.
If you go to that directory, move the "Outlook Express" folder someplace safe before formatting the drive. After you re-load your new operating system go into outlook express to the file menu and select "import".. then "import messages".. .then choose "outlook 6", and finally select "import mail form an OE6 Store directory" and then browse to the outlook express directory that you have saved wherever you put it.... it will suck all of that email into your new outlook express installation.
Hope that helps and that my explanation was clear enough.
-jd