If you switch frequently between a Mac and a PC, chances are you have to deal with a Windows keyboard from time to time. Thankfully, this can be easy with third-party utilities, or even features already built in to Mac OS X.
For most switchers, the hardest part about learning to use a new Mac is dealing with your muscle memory. For example, if you're really used to typing Control + C to copy something, Command + C means using your thumb instead of your pinky to perform the operation.
In System Preferences, you can click Keyboard and Mouse to change how your modifier keys (that is, Control, Command, Option and Caps Lock) work. Click the Keyboard tab, and then click the Modifier Keys button at the bottom of the window. You can map the Control key to the Command key (and vice versa, if you prefer) to help ease you in to Mac key commands.
(Not that we condone such activity, but this makes for an easy April Fool's day prank, too.)
If you need more control (for accessibility reasons, for example), you can use a free utility like DoubleCommand or KeyRemap4MacBook. With DoubleCommand, you can remap keys to the number pad, and make use of multiple-key combos, like shift-backspace for forward delete.
KeyRemap4MacBook lets you specify which keys are remapped to other keys, rather than selecting from a predetermined list. Some remappings aren't possible (due to the way Mac OS X works), but this is an even more flexible alternative to DoubleCommand.
Also -- don't use DoubleCommand and KeyRemap4MacBook at the same time, as they use the same methods for taking over your keyboard. Dogs and cats living together: mass hysteria.
If you're looking for keyboard macros -- that is, executing a series of keypresses by typing just one key -- products like QuicKeys, Keyboard Maestro, and iKey are for you. Windows-centric applications like Parallels, VMWare or Microsoft's Remote Desktop Connection tool all offer built-in command key mappers to ease the challenge of bi-OS living.
Now if only I could get a keyboard for my feet, I could be twice as productive.