As this has just happened, I suggest you run System Restore in Safe Mode, if it is enabled.
Restart the computer and start tapping the F8 key, select the Safe Mode with Command Prompt option. If you are asked to select an operating system, use the arrow keys to select the correct one and press Enter. Log on as an administrator or with an account that has administrator credentials.
At the command prompt, type %systemroot%\system32\rstrui.exe and then press Enter. Follow the prompts to restore the computer to an earlier date. You can, of course, run System Restore again and choose a different restore point if one exists.