How do I boot from an external USB drive (not a thumb drive)?

I bought a new PC recently, which came with Windows 7 pre-installed. I want to run Linux, but I don’t want to just throw away Windows because (1) I paid for it when I bought the PC, and (2) it might be useful to have around on occasion. I had planned to install a swappable drive bay, but that plan was thwarted by the design of the computer that the drive bays are behind little spring doors. And since Windows is already installed, it’s too late to partition the drive and set up a dual-boot scheme (and I’m afraid I’ll run afoul of Microsoft’s copy protection if I uninstalled it and tried to reinstall it). So now my plan is to install Linux on an external USB hard drive (a real drive, not a thumb drive). So is it possible to boot from an external USB drive?