Boot Windows from usb on mac

My situation:
- I have an installation disk of windows Xp/7 and of Mac OSX 10.5.8
- I have an external harddisk (320GB, USB)
- I have an iMac (PPC G5)
- I have a windows pc (Xp/7)
- I want to install windows on the external harddisk and boot it with my mac (without bootcamp).
extra: Will the windows run slow ? (cuz it's USB)

Can you guys help me ?!

yeah...i figured that out after some googling (is that a verb?).
sad enough...