Black screen after Windows 7 install on late 2009 iMac

Boot into MacOS, install either NTFS Mounter or NTFS 3G in order to make the NTFS partition writable.

On the Windows OS drive, delete the existing ATI driver:
/Windows/System32/Drivers/atikmdag.sys

Reboot into Windows 7, download the latest ATI HD 4850 driver from AMD.

Note: Appleā€™s alleged fix for this graphics card driver issue is a dud.