If you have created a System Restore Point, go to Start > All Programs > Accessories > System Tools > System Restore and select the Restore my computer to an earlier time option. Click Next, select a restore point and follow the provided steps. If you don't have System Restore Point, try using your Antivirus to scan and remove all malware, viruses and spyware from your system. If you don't have an Antivirus program, I suggest you to use the troubleshooting guide provided by Bleeping Computer. Also, try performing a Disk Check. Right-click on the Windows drive, select Properties and go to Tools tab. Click the Check Now button below the Error Checking option, click the Automatically fix file system errors and Scan for and attempt recovery of bad sectors boxes, press Start and follow the provided instructions. As a final solution, you will need to reinstall your Windows operating system.