ok, now that we've got some information, here are the steps.
1. Backup the server.
2. Verify that the backup worked fully.
3. Take another backup.

4. Check that all devices on the server are on the Windows Server 2003 HCL which will need to include the SCSI / SAS / fibre controllers for arrays.
5. Ensure that you have considered all effects. OWA, firewall, Blackberry, bridgeheads, backup software versions.
6. Will the backup software work under Windows 2003.
7. Get hold of the media - Windows Server 2003 disk and service pack 2. Preferably on the hard drive as it is a lot faster. Also have a copy of Exchange 2003 media kit and service pack 2.
8. Have a fall back plan!
9. Kill firewall to Exchange link (OWA and inbound emails) - minimise surface attack opportunities. If you do this before the backup then you won't lose anything.
10. Run the Windows install.
11. Install Windows SP2.
12. Windowsupdate.
13. Test a backup.
14. Telnet check of Exchange (locally).
15. Get the bridgehead delivering email again and allow OWA in to the building.
16. Disable any services you don't need.
17. Read all event log entries applicable.
18. Update system documentation and change control systems.
19. Send an email to all users notifying them about changes.
20. Kick back, drink coffee.
I have just checked my notes from the last time I did it and that's pretty much it. 4 and 6 were critical for that server...