Linux vs Windows 7 Web server performance differences
For all responders who suggested to use Windows Server 2008...
It was AniVirus. It was not disabled properly.
Once it was disables the performance improved from 600 req/s to 5000 req/s - not as good as Linux but at least reasonable.
I'd be more interested to see a Comparison with Server R2. Windows 7 shouldn't be running web servers, though I agree the difference in results seems extreme.