I don't think I have. It was brand new installation. When I turned it on, the windows just finished installing itself.
It is strange, though.
I was still trying to push F11 but that screen actually appears so fast that I think it doesn't have the chance to react to the command.
// I think I solved it!!
I inserted Windows 7 64bit recovery Disc. I restarted NB and when the screen appeared I chose the first option as always, but then
pushed F8. Then I chose "Repair your computer"
I selected "Command Prompt" and typed these:
I hit Restart and it works fine! I'm so glad..
I'd like to thank to everyone for your help!