well,you have to use some boot media,e.g USB Flash Drive(making it bootable), burn the XP ISO to the Flash and than boot from that, For this Why not try unetbootin-windows-248.exe ? I find this program very helpfull in making different types of installation "cd`s" on a usb disk Works fine with all iso, img files that I have tried (a lot)
You can find the file on
http://launchpad.net/unetbootin/+download
Just start program, add ISO file you want to "burn" to usb disk, and it just works !