You should try to boot the computer with a boot disk or usb that has anti-virus software built in. This will start the windows machine and will scan the hard drive for trojans. It will find & remove trojans which may solve the problem. Read More - http://www.howtogeek.com/175234/how-to-use-an-antivirus-boot-disc-or-usb-drive-to-ensure-your-computer-is-clean/
Else, you can format the hard drive ('''only if data is unwanted''') to erase the infected drive content. Reinstall the OS and use machine as usual. But if you can't afford to loose data then try to boot the system with DVD mounted with anti-virus. If possible also try to run hardware test to diagnose any H/W issues.