Boot using External HDD

it is a long and complicated process

this is a good guide:

http://www.ngine.de/index.jsp?pageid=4176



there is also a good thread in the forums with more mac specific info.

EDIT: found the link:

http://guides.macrumors.com/Booting_Windows_XP_From_An_External_Drive

there it is.

oh yeah, and it also involves opening up your computer and removing the hard drive. that is the only reason why i'm not doing it. but i REALLY REALLY REALLY want to. i just don't have enough space on my internal drive for a windows partition...