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.