As a general rule when working with badly infected systems, the only guaranteed way to get it working again is to nuke and reinstall. Symantec is, I suppose, doing them a service by trying to help, but if a system is too far gone to allow Norton to be installed, it's most likely too far gone to save.
Every client of mine that insists on going through the motions about not wanting to pay for a data backup or to get all of their programs reinstalled signs a waiver that states that I offer no warranty on the system or any of the work done on it.
Surprisingly enough, most of the systems I get that are so bad that they require a nuke and reinstall were running Norton to begin with.