Ubuntu 12.04 freezes at login unless Windows was started first

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http://askubuntu.com – I have bought a Toshiba Laptop Satellite C665D-M5010. the details are here: http://www.toshiba-india.com/laptop/satellite-c665-amd-viewmodel.html I have installed Ubuntu 12.04 which freezes on login screen. The system is dual-boot with Windows 7 on it. If i start Windows first and then restart to login in Ubuntu everything works fine. I am not able to find out any solutions. Please help me out. (HowTos)