The easiest way to try Linux without repartitioning your hard drive (or adding another) would probably be to use a distribution that boots directly off of a CD. Some of these can use a USB key drive to store settings.

CD-based Linux Distributions and Live Linux CDs


Some versions of Linux can also be installed to a file on your hard drive, so no repartitioning is needed, though this does require a bit more elabourate setup and tweaking. In Mandrake, for example, there is an option in the partition utility (advanced mode) to create a file and use it as a loopback device (the file acts as a virtual Linux partition). Near the end of the install there is an option to set up a boot manager, which you can have written to a floppy (assuming you have a floppy drive) rather than your hard drive, so you can boot the computer with the floppy inserted to either get a boot menu or go straight to Linux, or without the floppy to boot into Windows.