Changing SkyrimPrefs

I recently got the razor1911 cracked version of skyrim but i don't have a very good computer for games. So I went onto youtube to look up how improve my games performance and found a video about changing things in the SkyrimPrefs file.
And when I went to save my changes it said:

cannot create the C:\ Program Files (x86)\ The Elder Scrolls V Skyrim\Skyrim\SkyrimPrefs.ini file

does any one know how to fix this?