Possible? yes. Why would you want to; it would run so slowly? That's not a rhetorical question; for instance, I have a thumb drive that can boot five different repair / diagnostic environments.
If you're thinking about running more than one computer off the same copy of XP by using a USB drive forget it unless the computers are identical in every way and can be kept that way for ever. XP adapts itself to the environment in which it's installed. Moving the OS from one environment to another would cause major problems. More problems than the cost of a new OS, especially XP which I'm sure is cheap right now.