Sounds like you have a virus...
First thing to do is go to another computer and follow the instructions at
http://windows.microsoft.com/en-us/windows-8/create-reset-refresh-media to make a new installer using USB or DVD. You will also need an external hard disk to copy the files from your computer.
After backing up, you will put in the installer and follow the instructions for a "clean install". Once you install windows, buy and install a proper antivirus solution (kapersky, bitdefender, norton).
ONLY AFTER SCANNING the external drive, you can copy your files back in, making sure to delete anything that had infections.