install ubuntu 11.10 onto usb

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http://askubuntu.com – this is my problem. i inserted the ubuntu cd while im on windows xp. and my flash drive was pluged in. now a ubuntu window opened with 3 choices. i selected to install ubuntu. and the location to my usb stick. then it installed successfully. now when i reboot the pc it ask which os to boot. then i selected ubuntu. then in a black screen with GRUB> so how can i go ahead, please help. (HowTos)