This is the tutorial to downgrade the OS X Lion.
If you have only had problems with this new operating system, Snow Leopard would not go bad.
I solved the problem as follows, I hope it might help others out in the same situation:
1) Boot into OS X Lion
2) Insert your OS X Snow Leopard DVD
3) Applications / System Preferences / Startup Disk
4) Click your Snow Leopard DVD in the list and click Restart. Your Mac will now reboot fully to the installer, erase the partition and install!

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