click on the start button thingy, type CMD in the start search box at the bottom of the start menu, then right click on the icon for CMD that will show on the start menu and select "run as administrator" (enter your password if needed)
Once the command line window has opened type into it "sfc /scannow" and hit enter
Windows will now scan all important system files and restore any that have been changed back to default.
once completed run a full scan with your anti virus software.