This problme could occur if any other user is connected to Backup Exec server through Terminal services. The user currently at the console running setup will not be able to see this. If this is the case, then refer the following technote to resolve the issue:
http://support.veritas.com/docs/244758
Furthermore, view all processes for all users by selecting the "Show processes for all users" option at the bottom of the "Processes tab" in the Windows Task Manage, then end all the processes related to Backup Exec and verify the result.