Solution (step by step from the beginning):

1. Run Boot Camp Assistant from Mac OS X Snow Leopard (Applications -> Utilities)
2. Create a partition for the Windows installation using Boot Camp Assistant
3. Plugin your install disc and install Windows 7
4. The system will reboot a couple of times during the installation
5. The problem: The second time the system reboots, you the see the Windows red-green-blue-yellow animated logo, and then the screen goes black and hangs infinitely.
6. Press and hold the power button on your iMac until it shuts down.
7. Start the iMac again holding “alt” key, so you can select and load Max OS X Snow Leopard again.
8. Run any application that let you work with NTFS partitions (e.g. Paragon NTFS for Mac OS X or MacFuse)
9. Find the file “C:\WINDOWS\SYSTEM32\DRIVERS\ATIKMDAG.SYS” on your BOOTCAMP partition, and simply delete it.
10. Restart you iMac, and boot up Windows 7
11. Now Windows 7 should boot normally
12. Run update windows utility and install Boot Camp drivers from the Mac OS X install disc