Ubuntu stuck on login screen and it goes black

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http://askubuntu.com – I recently install the latest version of Ubuntu and it went perfect. Since that is stuck in login screen and I can't even type anything from keyboard and use my mouse to do something. Then after 5 mins. it does black like its in idea mode and stay as same condition for while. I have to turn it off manually and on to get to my home screen or login screen. I tried Ctrl+Alt+F1, and e key but nothing works. Pls, help me out. And I would like to install Windows XP instead of Ubuntu, is it possible to install in this mode, I tried once but does not work so, I think I need to start Ubuntu first then (HowTos)