Usually it's in the display properties. You can reach them via Control Panel, or with a right click on an empty part of the desktop. You can reset the settings for the resolution to what it was (like 800x600 or 1024x76. On that screen the OK-button is the default button, so you press the enter-key for OK.
Or boot into Safe Mode, and do the same, if you can't get it working in Normal Mode. In Safe Mode, all buttons are supposed to be viewable.
If all else fails, boot into MS-DOS, type scanreg /restore (followed by enter) and choose a date (from the 5 available) when everything still worked. This will restore the registry, where all this information is stored.
Hope this helps,
somehow i accidently made my everything on my screen enlarge and i don't know how to change it back. Also i can't reach any "ok" buttons because the screen is so big.