if you alter the .wims themselves you maybe can't install service packs as the integrity (core) has changed. This was the case with Vlite and vista anyways, and when i used vlite on Win7 it worked, but i was left with only 3 days to activate, and 7loader said i had a different OS core. But, modifying the ISO is fine. All you need from the iso to install windows is technically the install.wim, wich you can install from a command line with imagex if you wanted to.