I have a ThinkPad with an ExpressCard slot. I have a small stack of 64gb "Wintec Filemate" ExpressCard SSDs, each with a different operating system installed. To boot my laptop I simply select one of these, insert it into my laptop and turn it on. All my personal files are stored on two partitions on the internal laptop drive.

Of course you don't need any special slots to use multiple operating systems. As another commenter mentioned, you can install different operating systems to separate partitions. I've noticed that Windows is much less picky about sharing a hard drive than it used to be, but if you are concerned you could get, for instance, a caddy to put a second hard drive in your CD tray, or use USB disks.