how can I reset a lost administrative password ,considering that Ubuntu and windows7 are both installed on the computer , considering also that "Boot up the machine,and after the BIOS screen, hold down the left Shift key." didn't logged me to any thing, just continue the loading of the system as normal??
"Boot up the machine,and after the BIOS screen, hold down the left Shift key." Did indeed NOT log you into the machine. That is correct. You are seeing a menu instead. If not, then don't read any further and try again.
If you do see the menu: Choose "Advanced options", then choose (recovery) from the menu. Then you see a lot of text passing upward on the screen. Do nothing. Then you see a colored menu. Choose "root".
mount -o remount,rw / passwd your_username_here
change your password