Ubuntu crashes at starting

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http://askubuntu.com – I have my hdd partitioned with Windows 7 and Ubuntu 12.04. Windows runs normally, but my Ubuntu started crashing yesterday. Basically, when the username+password window has to appear, the screen turns black and starts flashing (but most of time is black). I could open a shell with Ctrl+alt+F1, but I don't know what to do, I think is something related to graphics. I tried something I read about purging nVidia drivers, but It didn't work. I took a photo in blast-mode in order to read what is going on when the screen is flashing. This is what it shows if you can read as fast as Superman: http: (HowTos)