how to install openSUSE 11 on a usb external hd and make it work on another computer that has Windows XP only?
You would be better off installing it on the target machine from the beginning for one thing. Otherwise carrying the disk over, the OS will encounter hardware changes and give you hassles reparing it to adapt it to its hardware environment.
although this is for the factory builds, it might be what you need: openSUSE Lizards » Factory USB Images
you end up with a usb stick which after 1st bootup will be a normal working suse install, you can then install hardware drivers for your pc on it.