[ubuntu] want to run and install ubuntu from usb but it keeps prompting login

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http://ubuntuforums.org – Hi, I used to have ubuntu 10.04 I guess, but I forgot my password, so in windows 7 I have deleted the partition with ubuntu from diskmgmt.. Now I have made a bootable usb(instruction followed from the ubuntu.com website) with Ubuntu Desktop 12.04 LTS on it.. if I boot from usb and run ubuntu, ubuntu asks for a login.. i tried ubuntu an pass blank, didnt work.. I tried to change pass but also didnt work. if i press enter it shows black screen for 1 sec and then asks for login again... Someone know what to do now?? (Hardware)