If it’s possible what would be involved? I assume dual-booting would be necessary. Would a USB2 hard disk be fast enough for this purpose?
If not an OS, then would I be able to run a game, or other software?
In terms of operating systems, I think that depends on your BIOS and its ability to boot from USB devices.
As for games and other software, it depends. Some software can run straight out of a folder – in that case, you can definitely run it directly from the USB drive. Other applications, however, require entries in the Windows Registry (normally put there during installation) and will not run without them. You can export the settings to the USB drive and import them on any computer you want to run the program on, but that’s not really an ideal solution.
Oops, I assumed (mistakenly, perhaps) that you wanted to carry the drive around and attach it to other computers so you can run programs on them.
If that’s not the case and all you want to do is use it as a permanent external drive on your own computer, feel free to install programs and such to it. Windows will treat it the same way it treats any other hard drive. The speed might be a bit slower, but probably nothing too terrible if it’s over USB2.
That was a quick uh… Reply. Thanks.
If I were to install a game or software on the drive, and only run that software/game from the computer which it was installed from, it would work even with registry entries right?
I don’t intend to transport the drive to other computers and run the software there, I merely want extra space with the added convenience of portability, so I could install space-hungry software there (games) and place other space-hungry stuff there (with 7MB a picture my photography hobby is eating up space fast). I’d only ever run the games on the one computer.
My bios/computer is quite modern. It’s an alienware laptop bought new about a year ago.
And your second reply addressed my second reply (simulpost) Thanks for the help again 
You can certainly do it (I’ve got my USB keychain set up as a bootable hard drive, so long as the BIOS allows it it’s a very cool “rescue” device).
The thing I’d be careful of is that the drive letter your machine assigns to your USB hard drive shouldn’t change, that will definitely cause problems with installed apps that are looking for D:\windows when those files now reside on E:\windows, for example.
On preview, it’s been mostly answered, but anyhow:
Microsoft provides a page on requirements for booting via USB here, though they’re kinda fuzzy on how to actually do it.
If you want to boot Linux, google “usb disk boot linux” or “usb key boot linux” (for booting from USB keys) and you’ll find lots of information.
Provided the disk numbering didn’t change, Windows should have no problem installing games to a USB device, thought I’d prefer the faster-type USB connection lest I get fragged due to USB overhead. 
I must be missing something here. Why not install your games/OS on the local hard drive, and store your 7MB pictures on the USB drive? Or better yet, burn the pictures to CD’s or DVD’s? Then you could put the CD/DVD into a (relatively new) DVD player and have a slide show.
(Or did the need for more storage space just lead you to this hypothetical question?)
Mostly the OS question is hypothetical (it would just be cool to have a ‘portable OS’), but I am running short of storage space and It’s got to the stage where I have to uninstall one game to install another.
I also like the idea of having my entire music collection all in one place. And I prefer to use the computer to play it so why not put it all one one huge transportable hard disk.
Storage keeps getting cheaper - a 250GB hard drive is about $160. At some point you’ll find it’s just a lot easier to replace your existing hard drive with a great big new one rather than adding on external drives.
And while you can’t do it with Windows now, you can make Linux distros that run 100% of a CD or DVD (e.g. Knoppix). Every computer out there will boot from the optical drive (so no worries about does the BIOS support booting from a USB drive) and you aren’t even touching the actual hard drive. Voila, turn any machine into a gaming console and all you have to do is carry a CD with you.
This may be a hypothetical question, but it will come up when this topic is search, so I present to the members of the audience who don’t feel like googling:
Jeremy Wagstaff’s A Directory Of Programs Designed For USB Drives and other non-tragic events.