That file is
not read only, so there should be no problems whatever, unless you have installed your game on a read only device like a CD. But then you would have other problems...
!
A Simple right click on the file and examining its properties should show that. (And allow you to reset it from read-only if for some reason you have managed to change the attribute - sometimes restoring up from a CD burned backup does that (better to backup into a zip file if putting any backups onto a CD - you can then extract from the archived ZIP or RAR and so avoid having files tagged as read-only)).
Of course, you may be using Windows vista or Seven, and perhaps that operating system thinks you need admin privileges to alter an ini file?. If you have Windows beyond XP, then you may have to read your Windows help documentation. (I know that Seven does some darned strange things to files if placed into the root of C:\, even if you then later move them. And that was just a .TXT file I had copied over temporarily. So avoid the C:\ root, as it seems to thing anything put there is an Admin file! - *!*g nanny-state operating system!
!)
Cheers
Andy