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Do Option 2 of this tutorial SFC /SCANNOW Command - System File Checker
it will not harm any data or programs you have installed it will try to find and repair any corrupt system files that you have. You may need to do this up to 3 times with a reboot after each scan.
Be sure you are using the 64 bit version in either tutorial below.
Try 1 of these 2 ways of Enabling custom themes for it to work. Enabling Custom Themes - Windows 7
Or Theme - How to Apply Custom Themes