Booting vista from external HD?

I'm not exactly sure where I should post this so here goes...

I have very little experience in this and I am just wondering if it was possible. Is it possible to install an OS (In this case windows vista) on to an external HD, and simply stick the usb plug into any computer and have the computer boot from that? Would there be any hardware issues / slowdowns from any limitations?

Basically I would be able to carry my OS and my files around with me, and could basically plug it into any computer and load and run all my programs. All traces of my activity would stay on my hard drive, not on the computer I'm using. For example, I could have some video game or something installed onto my OS, then I could just go and stick it any computer, have the external HD boot up, and play that game, simple as that, on any computer?

Is this is anyway possible or feasible?

Thanks for the help.

I 'think' it's possible if you're only running it on one machine, but you're motherboard needs to support booting from usb. If you're switching it between computers, you're going to run into driver problems. Even if it does work, it's going to be noticeably slower than running off an internal drive.