Since Windows 98, a feature has been included for changing the buttons on your mouse to help you lefties out there. This feature allows you to replicate the feel of the right-handed mouse (like having the primary mouse click be your pointer finger).
To switch buttons on your mouse to a left-handed configuration in Windows 98 and ME, go to Start, Settings, Control Panel and double click the Mouse icon. On the Buttons tab, choose the left-handed setting under Button Configuration.
For Windows XP users, go to Start, Control Panel, select "Printers and Other Hardware" (if in category view), then Mouse. Go to "Button Actions" and change the Left button action to "Right Click" and the right button action to "Left Click." (Don't be surprised if your user interface looks different. Just check things out, the feature should be there.