For Windows XP
Find the folder of you LoL
Look for file that called "game.cfg"
Then open it with Notepad.
Look for this
If found it,change it to your maximum resolution.
How to know your resolution?
Go to desktop then Right-Click-->Properties--->Setting TAB--->Change it to Maximum
Example 1600 x 900 then click ok
Change the game resolutions same like your computer resolutions.
Hope it help : )