In order to resolve this issue, remove any of the following registry keys which store information about the SID settings
HKLM|SOFTWARE|Microsoft| Microsoft SQL Server|MSSQL.X|Setup|SQLGroup
HKLM|SOFTWARE|Microsoft|Microsoft SQL Server|MSSQL.X|Setup|AGTGroup
HKLM|SOFTWARE|Microsoft|Microsoft SQL Server|MSSQL.X|Setup|FTSGroup
HKLM|SOFTWARE|Microsoft|Microsoft SQL Server|MSSQL.X|Setup|ASGroup
On a 64 bit Operating system the location of the registry key
HKLM|SOFTWARE|Wow6432Node|Microsoft|Microsoft SQL Server|MSSQL.X|Setup|SQLGroup
HKLM|SOFTWARE|Wow6432Node|Microsoft|Microsoft SQL Server|MSSQL.X|Setup|AGTGroup
HKLM|SOFTWARE|Wow6432Node|Microsoft|Microsoft SQL Server|MSSQL.X|Setup|FTSGroup
HKLM|SOFTWARE|Wow6432Node|Microsoft|Microsoft SQL Server|MSSQL.X|Setup|ASGroup
Note: In these registry subkeys, MSSQL.X is a placeholder for the corresponding value on a specific system.
Warning: Incorrect use of the Windows registry editor may prevent the operating system from functioning properly. Great care should be taken when making changes to a Windows registry. Registry modifications should only be carried-out by persons experienced in the use of the registry editor application. It is recommended that a complete backup of the registry and workstation be made prior to making any registry changes.
Run the upgrade of Backup Exec again to confirm that issue is resolved
For more information on the resolution above, refer to Microsoft KB article:
Error message when you try to install a SQL Server 2005 service pack or a SQL Server 2005 hotfix package: "Error 29528. The setup has encountered an unexpected error while Setting Internal Properties" http://support.microsoft.com/kb/925976