Cannot shutdown, computer freezes

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http://askubuntu.com – I've installed Ubuntu 12.04 (near windows 7) on my laptop(Dell studio xps 1640 with Ati Radeon HD 3670) and everything is working fine, but when I want to shutdown my computer it freezes! I don't know what to do after that, then I have to use power button to power off the computer and it's not good at all. I don't know what's the problem and I don't know how to figure this out. I installed the graphic card driver (12.6 version) from ATI website and it fixed my problem with playing videos but I think after that this shutdown problem appears! Would you tell me what's the problem and how can I (HowTos)