Hi Bill,
Since you are updating Backup Exec software with service pack, there a few other files which are required to be updated during this process. Hence update install would ask you to insert the Backup Exec installation cd.
When prompted insert the Backup Exec cd and point to the location winnt/install/be folder for the file "Veritas backup exec for windows servers.msi". Click on OK.
The install would then go smooth.
For any further queries you can contact me on mail : aatiquequreshi@walla.com
Regards,
Atique Qureshi
VCP, MCSE