Ever had a friend who wants to use your computer, but they are not Mac-savvy? Perhaps even *gasp* Windows users? Here are some steps to take to prepare for a situation like this.
First, set up a new account for "Guest". In that account, set up shortcuts on the desktop to anything that a guest may want to use -- most likely Firefox, and (God forbid) MS Word. Remove everything from the dock and hide it. Docks don't make a lot of sense to PC users, who are more familiar with shortcuts on a desktop or navigating a clumsy "Start" menu. No doubt you will also appreciate not having your own shortcuts and personal files on the desktop.
Quick Tip: a quick way to create a shortcut (also called Aliases) is to ALT-CMD-drag your application icon to a new location.
Next, open Finder Preferences. Uncheck Hard disks, connected servers, and spring-loaded folders. The first two are to simplify the desktop, the last so Finder behaves more like a PC.
Change the Desktop Picture to one of the default Apple images that have been around for a while. The new Windows Vista wallpapers look very similar to these, so the PC user will feel more at home.
Finally, a lot of PC users are very upset over the misconception they have that Macs are not capable of right-clicking. Rather than explaining CTRL clicking, you may want to buy a cheap two-button mouse for them. Don't try to explain the virtues of having an additional modifier key for keyboard shortcuts. Trust me. PC users LOVE to right click. Let them hold on to that, so when they are finished with your Mac, they will still be able to enjoy their own computer.
Be sure you have a password on your primary (presumably admin) account, to prevent any anti-Mac hacking by malicious guests.
Finally, you should enable fast-user switching. Open system preferences (under the admin login), and go to Login Options. Check off Enable fast user switching. I like the icon setting because it uses the least room in the menu bar. Authenticate, and you're finished!