You can't nlite an existing installation of windows. It is written to modify the contents of the setup package, giving you the choice of what to have installed and what not and also lets you add hotfixes, add-ons, etc. Therefore you can't install a nlited windows on the same partition with an existing windows already present (by default, windows will install in a different folder should one named Windows exists on the target partition). You can try to install it on a different partition if you don't want to format the existing installation.