[ubuntu] can't install ubuntu 12.04 after install xp

hi everybody,
i try to install ubuntu 12.04 on my pc and i have install win xp first. but i can' boot via usb stick,
i got a message when i boot ubuntu from usb stick :
"grub4dos 0.4.5c 2012-04-13..bla-bla-bla..."

please help me, i really want to using ubuntu again :D