You don't have to purchase a different mouse if you're left handed. Windows  recognizes that not everyone favors the right hand.

Follow these steps to turn your mouse into a left-handed device:

1. Click Start > Settings > and select Control Panel.
2. Double-click the Mouse icon.
3. On the button tab under Button configuration select Left-handed.
4. Click Apply.
5. You just toggled your mouse buttons to reflect a real left-handed mouse.