You will want to backup your PST and/or PAB file. (The "Contacts" folder is
stored in the PST file. The PAB is for the legacy Personal Address Book
service which you make or may not be using. See Tools > Email Accounts >
View/Change existing directories and address books. If the only thing
listed is Outlook address book, then you will not have a PAB file.)
Location of files are under:
\documents and settings\<userid>\local settings\application
data\microsoft\outlook
Outside of that,
http://www.slipstick.com/config/backup.asp is a good page
to learn about the different data file types Outlook uses. Might be helpful
to you in determining what to backup.
/neo
PS - Don't forget to take a peek at Office/Outlook 2003's Save My Settings
wizard. While it doesn't backup the PST/PAB, it will backup most of your
Office settings so they can be restored later.