Eh, how to say that - if you tamper with LocalSystem account, you should know what are you doing. The installation program on NTs needs high privileges - just to write to ‘Program Files’, to play with services, etc. It doesn’t need high user’s privileges, but it needs high LocalSystem privileges - the default are high enough, no new special ‘avast_installer’ user is not created. And, if some user deletes the privileges, installer won’t run at all.