Screen Resolution
Most medium to low end projectors are rated at SVGA (800 x 600) resolution. (If your projector has an XGA rating, there may be no need to reset your laptop's resolution.) You may be running your notebook in a higher resolution (e.g. 1024 x 768).
Your best choice is to re-size your desktop to 800 x 600. To do this, right click on the desktop then choose properties.
From the tabs in this window, choose Settings.
At the bottom of the settings window slide the screen resolution indicator to 800 x 600. When asked if you want to keep this resolution, answer yes. (Your computer may have to reboot.)