You may recall that one of Apple's slogans when the Mac mini was released was BYOKM-bring your own keyboard and mouse. Well a lot of those keyboards will of course be Windows keyboards, and while they work fine on a Mac, there's one particularly annoying thing. For some reason the keyboard Windows key is mapped to the Mac Command key and the keyboard Alt key is mapped to the Mac Alt or option key. The reason this is a problem is that on a Windows keyboard the Alt key is right next to the spacebar (where the Command key is on a Mac keyboard). So if, like me, your keyboard shortcut muscle memory is to the key next to the spacebar then all your shortcuts get messed up on a Windows keyboard. Fortunately, there's a simple solution to this problem in the Keyboard tab of the Keyboard & Mouse Preference Pane. There if you click on the "Modifier Keys..." button you'll be taken to a dialog where you can easily remap the keys. So to make a Windows keyboard work like a Mac keyboard just change the option key to the command key and the command key to the option key as follows:
You'll also note that the Modifier Keys dialog lets you take care of another annoyance, the anachronistic Caps Lock key. Personally I find Caps Lock worse than useless as I sometimes inadvertently hit it. So you can turn it off altogether as I've done here, or you can set it to emulate one of the other keys (as Unix and Linux aficionados sometimes like to do).
So if you like to use a Windows keyboard with your Mac for whatever reason, these modifier keys settings will help keep you sane. Since my preferred keyboard is an old-school buckling-spring EnduraPro 104, these settings have been a great boon to me.