1. Go to your desktop and right-click on some free space. A grey menu box will pop up.
2. Left-click on Properties near the bottom of the menu. A Display Properties box will pop up (see image).
3. Select the Settings tab in the Display Properties box.
4. Use the little slide bar near the lower left of the Display Properties box to adjust desired screen resolution and then click Ok.

After you've done this, Windows will ask you to confirm that changing your resolution is indeed what you want to do. After you've clicked Ok again, your monitor will flick off for a few seconds, it is normal, and then pop back to life again with the new resolution. Windows will usually ask you to confirm the change one last time, and then it will leave you to it.