The System Restore tool in Windows 8 is a very valuable utility when you're trying to solve a major problem.
Most serious issues in Windows 8 are caused by problems with the Windows
Registry,
device drivers, or
DLL files. When a restore point is created, a current copy of these and other important files in Windows 8 are saved and can be restored using System Restore.
Considering the scope of protection that System Restore provides, using the tool early in your troubleshooting could save a lot of time.Follow these steps for reverting important Windows 8 system files back to a previous state using System Restore.
from
control panel click and open the Windows firewall option.or if you have small icons click on System .
Click on the
System protection option
On the System properties window choose System protection. and click on
System restore
Click
Next 
Any restore points that you created, scheduled restore points that Windows 8 created, and those created automatically during the installation of certain programs will be listed here. You can not use System Restore to undo Windows 8 changes to a date that a restore point does not exist. So
Choose the restore point that you want to use and click
Next.
Click
Finish on the Confirm your restore point window to begin the System Restore.
Windows 8 will shut down to complete the System Restore so be sure to save any work you might have open in other programs before continuing.
Click
Yes to the Once started, System Restore cannot be interrupted. Do you want to continue? dialog box.
If the problem still persists, you can repeat the steps above and choose another restore point if one is available.If this restoration caused a problem, you can always undo this particular System Restore.
source:fullwindows8
