Looks like a script is run to cleanup the old agent but since it can't find the old agent the install fails. It should just continue on.
Action start 12:52:07: InstallFinalize.
MSI (s) (30:70) [12:52:44:073]: Product: Altiris Inventory Agent -- Error 1720. There is a problem with this Windows Installer package. A script required for this install to complete could not be run. Contact your support personnel or package vendor. Custom action DeleteInvDataMDB&RenameOld script error -2146828235, Microsoft VBScript runtime error: File not found Line 28, Column 2,
Error 1720. There is a problem with this Windows Installer package. A script required for this install to complete could not be run. Contact your support personnel or package vendor. Custom action DeleteInvDataMDB&RenameOld script error -2146828235, Microsoft VBScript runtime error: File not found Line 28, Column 2,
Action ended 12:52:44: InstallFinalize. Return value 3.
Info 1903. Scheduling reboot operation: Deleting file C:\Program Files (x86)\Altiris\Altiris Agent\Agents\Inventory Agent\TBDEE7A.tmp. Reboot to complete the operation.
Action ended 12:52:44: INSTALL. Return value 3.
MSI (s) (30:DC) [12:52:44:283]: Product: Altiris Inventory Agent -- Installation operation failed.
MSI (s) (30:DC) [12:52:44:283]: Windows Installer installed the product. Product Name: Altiris Inventory Agent. Product Version: 7.0.1278.0. Product Language: 1033. Manufacturer: Symantec. Installation success or error status: 1603.
=== Logging stopped: 10/19/2010 12:52:44 ===