How to map/bind mouse button as keyboard button in Windows 7?

Try X-Mouse Button Control:

X-Mouse Button Control is a Windows application to remap your mouse buttons and expand the capabilities of your mouse!

The extensive list of functions available to map to your mouse buttons includes options such as:

• Send any keys using the simulated keystroke editor
• Launch any windows application
• Copy/Cut/Paste
• Media control (play/pause/stop/volume/mute etc.)
• Capture screen (or active window) image to clipboard
• Click-Drag [Sticky buttons/keys]
• Vista (and Windows 7) support with some Vista/7 only features such as 'Flip 3D'
• And much more!