The command will work if you run it from the recovery system thats present on Windows Vista & Windows 7 installation media, yes. That said, it should also give you the ability to do it via GUI (somewhere, been years).
Not if the system includes Windows Vista, or Windows 7 (or a Linux bootloader). All of these create a separate partition that houses solely the bootloader and basic diagnostic tools, allowing any operating system partition to be removed (or added).edit: if you multiboot on the same disk, with different windows, does this mean that removing the one you installed first will always cause problems?
This is the "System Reserved" partition at 100mb you may have seen in Disk Management.