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I'm on my XP machine at the moment, so I can't give you good "directions", but I can give general suggestions. If you go to the control panel, and find Monitor, or maybe Display, and from there find the setting for resolution, it should have, I'm guessing at least 3 different resolutions to choose from. I would guess the middle one will be the default.
I think there are separate settings for screen width and height, which I would not touch. But if you did change them, look and see if there's a button to restore the defaults.
When I get back on my Win7 machine, I can probably find some more things to "look at". And other members may have some different ideas
The default resolution will probably say (Recommended) beside it.