I havent seen to many computers come across my bench that will boot froma USB key though hesperus. Allthough I am not saying you shouldnt have a copy of bart pe on hand. I use a variation of it called the ultimate boot cd for windows (recently discussed on another thread).

I have a neat little laptop drive enclosure that I have a 40gb laptop drive in that it roughly the size of a normal PDA. I have all sorts of goodies on there. ISO images of just about every windows ever released. AV tools. Documentation. The CIA world factbook (thx katja). It is even USB powered for maximum portability. But it does take 2 usb ports to use. One for the data and one for the power. Not normaly a problem though.