The problem
Sometimes, when you hook up a laptop to a projector, the image looks fine on the laptop screen but terrible on the wall. Why is the projected image so bad?

It's because the laptop screen uses a different screen resolution than those typically used by projectors.

Many newer laptops use a wide-screen display resolution (1280x800 and 1440x900 are common settings), while most projectors used in classrooms and similar settings use a display resolution of either 800x600 or 1024x768.

The solution
To make a projected image display more clearly, set the display resolution of your laptop to match the projector. In most cases, 1024x768 will work best.

For Windows XP users, navigate to Start > Control Panel > Display, select the Settings tab and adjust the screen resolution slider to the desired resolution.

For Macintosh users, navigate to Applications > System Preferences, select the Displays icon and choose the desired resolution from the list provided.

Don't forget to switch your screen resolution back once your presentation is done!

For more information on computer display standards:
http://en.wikipedia.org/wiki/Computer_display_standard