It could be an invalid package location.
- Open the Symantec Management Console
- Enter 'Inventory Plug-in for Windows Package' in the search field (without the quotes) and search
- Click on the 'Inventory Plug-in for Windows Package' result
- Click 'Edit Package' on the left
- Confirm that the package location (C:\Program Files\Altiris\Notification Server\NSCap\bin\Win32\X86\Inventory\Agent Package) is correct
Note: If you installed to a drive other than C:\, sometimes the Inventory Plug-in for Windows doesn't figure it out, and is still looking for it on C:\ rather than, for example, F:. Correct the path as necessary.
If the drive specified is correct but the package isn't there, repair Inventory Solution using SIM.
Does checking/fixing the package path help?