Looking at the disk activity access to disk is using all your resources not nginx.
Here http://s633.photobucket.com/user/C0nw0nk/media/Untitled-5.png.html you see nginx itself is waiting for disk IO to complete, all processes are doing just about nothing other then waiting for the harddisk, the main waiting issue looks like it is writing to disk which isn't going fast enough.

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nginx for Windows http://nginx-win.ecsds.eu/