[xubuntu] Replacing Ubuntu with Xubuntu

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http://ubuntuforums.org РI have on my computer WinXp and tried to install Lubuntu . Now it says it is Ubuntu. I am trying next to install Xubuntu. The present Ubuntu(Lubuntu) is defective but I can log into it. When booting the computer and the BIOS-settings adjusted accordingly, the system however goes where I can choose either windows or Ubuntu. Not to USB-flash. As the USB flash does not start installing Xubuntu, I am wondering if I can use some command in Ubuntu to start installation from the stick? Any comments as to what I shöuld take into consideration? (Hardware)