Okay, if you actually wanted NF in a window then it can be done using a virtual machine (a window that contains, simply put, another operating system) most commonly used to run older operating systems on a new system to use old software.
You can have that windows, and NF would still be stuck in fullscreen but the 'fullscreen' would only be the virtual machine window.
However, NF still screws up badly and is so poorly coded that this would cause signifcant drain on system resources and could stress even the best systems.
If what you want is to log in to two accounts at once, well... buy another pc? :/