Saving your mail in Outlook Express is a pain. It's doable, and I've done it several times before a reformat, but it's temperamemtal. It may not work, even if you do everything right.
Here's how you do it:
First Step (tell Outlook WHERE to save you mail):
Open Outlook.
On the menu bar, go to Tools --> Options
Click on the 'Maintenance' tab
Click the 'Store Folder' button. Change the location to some new folder on one of your other partitions.
Tell it 'OK' as many times as necessary, then close Outlook completely
Second Step (tell Outlook to actually save the mail to the location you just specified):
Open Outlook.
On the menu bar, go to File --> Folder --> Compact All Folders
Close Outlook completely
IN THEORY, this compresses all of your stored messages and saves them to the partition/folder you specified previously. Open up that folder and make sure there are now new mail files there, and that the files have a non-zero size. Copy these files to a second location, 'cause you may want a backup copy before you're all done.
Third Step (Erase):
Reformat/repartition/reinstall
Fourth Step (tell the new copy of Outlook where your old mail is):
Repeat Step One, telling Outlook to use the same new partition/folder you specified earlier
Close Outlook, and re-open it.
IN THEORY, upon restart Outlook should look in your new folder for mail files, and open up any it finds, and generally look just like it did before you reformatted. In practice? Well, who knows. Maybe it'll work. Maybe it won't. Maybe it'll delete anything it finds in that new folder (you did back up those files, right?)
You'll need to re-enter all of your mail account info (servers, login/passwords, etc) and other such settings manually.
Good luck. You'll need it.
