As long as you can write on the filesystem you can do whatever you want with the system, on XP you can install a special crafted GINA DLL logging you in as any user you want, you probably can add a service running as LocalSystem in the registry and even get remote access to the machine this way. In linux you could replace init by sh and you'll be root. File system encryption will indeed be a problem, provided the user cannot find a way to get the key because if vista doesnt prompt the user for a key, its obviously stored somewhere.
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An easier way is to run a system service and tell vista to load it as LocalSystem
As long as you can write on the filesystem you can do whatever you want with the system, on XP you can install a special crafted GINA DLL logging you in as any user you want, you probably can add a service running as LocalSystem in the registry and even get remote access to the machine this way. In linux you could replace init by sh and you'll be root. File system encryption will indeed be a problem, provided the user cannot find a way to get the key because if vista doesnt prompt the user for a key, its obviously stored somewhere.