gnu grub 1.74 beta 4 - need step by step help for an ABSOLUTE beginner

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http://ubuntuforums.org – Hi. I got this computer from a friend, it is supposed to Windows and Ubuntu (don´t know which versions) , but unfortunately the instruccions she gave me don´t work, and she´s left the country, so I am kind of looking for my way in a total darkness... When I start the computer, I get this famous message "gnu grub 1.74 beta 4 minimal BASH-like line editing is supported. For the first Word, TAB lists posible command completions. Anywhere else TAB lists posible device/file completions.", that I´ve seen on other threads also, but the advices there don´t seem to work for me. Since (Hardware)