LCD screens have an optimal resolution - which is when the resolution you set it too exactly matches the number of pixels of the display. You will see this recommended resolution in the manual. If it's a 15" display, it's probably 1024x768.
Any other resolution is a compromise - some displays handle it well, some not so well. Sounds like you've got a "not so well one". Usually there are some controls for the display itself (look for the menu, or setup button on it) which you can adjust to improve matters. Often they're timing/phasing controls and the like.
That's one of the drawbacks of LCD displays, I'm afraid.
You're probably better putting the resolution back to standard, then in the display properties, adjusting the text/iconsize/dpi etc.