Assuming the full Outlook, not Outlook Express:
Open Outlook
Click File -> Import and Export
Select Export to a file
Select Personal Folder File (.pst)
Choose the folders you want backed up. Be sure to indicate if you want to save sub folders as well.
Choose a location for the file, and export.
Alternatively, look for the file "backup.pst", copy it to a safe location and format away.
Upon reinstall of Outlook, basically reverse the above, Import the file and all your emails, contacts, calendar info etc will be restored.
\Dan