ATI CATALYST didnt update correctly and now cant boot ubuntu

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http://askubuntu.com – i recently bought a barebone with Ubuntu 11.04 64 bits installed, which worked correctly except that the video worked a bit bad. So i tried downloading ATI Catalyst from ATI web, and tried to install. As Ubuntu asked me to uninstall first the old Catalyst or force the intallation, i tried forcing, and then after rebooting, the system freezes and shows a failure saying automatic crash report generation. Then i tried rebooting in safe mode, wich works correctly, and try to reinstall ATI, but shows an error at installation. Rebooting the system, it now show a black screen, and does nothing. Pleas (HowTos)