I've been lucky with 8.1 - I haven't run into any problems yet. I'm using an Acer 19" LCD monitor, but there are adjustments on all monitors you can use to widen or heighten the display. Unfortunately they're different on every make and model, so you'll have to examine your monitor and/or it's documentation to find where it is. You could also look at Device Manager and see what that says. If there's either a ? or an !, you need to download a driver from its manufacturer.
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Good luck.
I just downloaded the windows 8.1 preview and am baffled. The picture does not fill out my monitor. It recognizes my monitor, the resolution settings are correct but all I get is a 22" screen in a 24" monitor. Any idea of what to do? Video drivers? Monitor drivers? Any help would be gretly appreciated.

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