in windows xp, open windows explorer, go to c:\documents and settings\owner (or whatever account you installed the game under)\application data\natural selection 2\options.xml
edit it, search for those lines of code, and replace them with your desired resolution
Comments
heres the answer
C:\Users\"username"\AppData\Roaming\Natural Selection 2
and open the options.xml in an Editor.
Change the following lines to an resolution you want e.g. 1024 *768 or 800*600
<graphics>
<display>
<invertedmouse>false</invertedmouse>
<x-resolution><b>1680</b></x-resolution>
<quality>2</quality>
<y-resolution><b>1050</b></y-resolution>
<fullscreen>false</fullscreen>
</display>
</graphics>
edit it, search for those lines of code, and replace them with your desired resolution