My cab is on the network, and I have a Windows Home Server. So I back up the actual OS stuff, emulators, configs, ect. ROMS are not included in the backup, as I have complete backups of them on an external hard drive, my main desktop, my notebook PC, and a folder on the server.
The Home Server is not programmed to back up the cab every night (it would fail anyway since the computer is completely turned off when I'm done). I initialize the backup manually when major changes occur (Windows updates, major changes to front ends).