Cam,
I think you'll need to rightclick on the file you've downloaded (as you've installed winrar right? rar file is just another way of compressing files like the zipped files you see everywhere) and unpack the file to somewhere of your choice like your desktop for instance..
Then open the unpacked file and ignore the 'Play Tiberian Sun' icon as that didn't work for me and navigate to the main game folder. Look for the exe shown in the screenshot and rightclick on it and choose properties, then compatibility. Set it for 'Win98' and your good to go.. Click on the icon you've just changed and the game will start.
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