As for windows, at least Realtek wifi utility has a "Software AP" feature. Haven't test it though. In this case you just specify gateway adress on the PC = N900 adress and you should be done.

I think ASUS has the same option.
In my case it's Atheros under Win7, I use windows to maange wifi.

Besides, there have been rumors about finally implementing p2p mode to wireless adapters as standart.
http://en.wikipedia.org/wiki/Wi-Fi_Direct