You must install the .NET Framework version 2.0 SP2 before you install the Backup Exec Agent for Windows.
The .NET Framework is required to install the Agent for Windows. If the Windows Server 2003 computer on which you try to install the Agent for Windows does not have at least .NET Framework version 2.0 SP2, the installation fails.
Exit the Backup Exec Installation Wizard and install the .NET Framework on the computer. You can download the .NET Framework from the following link:
https://www.microsoft.com/en-us/download/details.aspx?id=1639
After you have completed installing the .NET Framework, you can run the Backup Exec Installation Wizard again.