I don't know which version of Outlook you are using so here is the steps for Outlook XP:-
1) Open Outlook
2) Click on File\Import and Export
3) Select "Export to a File" and click Next >
4) Select "Personal Folder File (.pst)" and click Next >
5) Select "Personal Folders" and then "Calendar" and click on Next >
6) Specify a name and location and click on Finish.
Once you have re-installed Outlook on your newly re-formatted computer, simply open Outlook and:-
1) Click File\Import and Export
2) Select "Import from Another Program or File" and click Next >
3) Select "Personal Folder File (.pst)" and click Next >
4) Specify the name and location and click Next >
follow the steps from there.
Hope this helps.