If you only need to backup your address book there are two ways to go about it. Either open outlook express and go to the file menu, export, address book and save it as a .csv file for easy inportation later or copy the .wab files directly from the hard drive. They can usually be found in the following folder
C:\Documents and Settings\****\Application Data\Microsoft\Address Book
**** being your username
If you want to backup all of your emails the easiest way is to copy the entire email store folder which can be found in the following folder
C:\Documents and Settings\****\Local Settings\Application Data\Identities\{AD51CEF9-9E6D-3HH7-8C4C-A7449878GMB}\Microsoft\Outlook Express
**** being your username
to find this directory you need to make sure hidden files and folders are viewable.
the large numbered folder in brackets after the "identities" folder could be named any string of numbers and letters but luckily you should only have one folder there as long you only have one email account in your current outlook express profile
Hope this helps and is not too long winded