1. Why would you want to?
1a. Presumably so you can boot into it from any PC?
1a solution: Obtain a portable version of Windows XP, don't attempt to install it to a usb drive (pretty sure you cant)
Warning: Windows XP is going out of support very very soon, please consider the security aspect.
Other than that if you can provide any more info like what you intend to do with it or what it will be used for or on and we may be able to be a bit more specific.
Hope this helps,
-Chris