I can boot from USB a installation of Windows 7, but I can't do this with Ubuntu or XBMC live

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http://askubuntu.com – I'm using a old pc with Intel Pentium 4, 1gb ram, 200gb HD All this night I tried to boot Ubuntu from USB... I can do this with Windows 7 normally I tried first using Startup Disk Creator, but freezes. I tried using UNetbootin, freezes too, choosing any option I tried CD, but continues the boot of windows the a I have installed So, I tried booting a Easeus Partition from boot, and work, and I format my disks normally But anyway, when I boot Ubuntu, don't work... I don't know what I do to install Ubuntu in my PC, I need help... Thanks P.S.: Sorry my english, I'm from Brazil (HowTos)