I am upgrading from SP21 to SP24. When I run the upgrade procedure for the client (client.msp) everything goes well, but when I start the client it still shows the old patch level and SD does not start.
you could manually copy the file from the service pack to the location George stated on the client system
You could use SMS to deploy the upgrade (or similar like Radia if you need to cover a lot of clients)
Have a look at the folder structure and see what permissions are on the folders where the SP files are to be installed. Have youa policy preventing such files from being installed that is being applied to the users systems?