You could buy a good anti-virus, or if that doesn't work you can factory reset the computer. Remember though, that will delete everything on the computer as if it had just come out of the box. Use that as a last resort. Good luck.
Restart in safe mode with networking….there download the free version of malwarebytes….install / update and do a full system scan…..after that is done….still in safe mode …go into control panel uninstall programs….look for programs install just when this happened ( date wise )….uninstall all of them…..
Also while still in safe mode….run your antivirus program….
After all is said and done…..restart your computer in safe mode ( this time without networking ) run malwarebytes and your anti virus again…..
Also….while in safe mode…..system restore / restore points….turn that off for now….when your machine is good and clean….remember to turn restore points back on and create a clean restore point….
Download from here http://www.avg.com/gb-en/avg-rescue-cd AVG rescue disk and burn it to a CD. You may need a working computer if yours is badly infected. Boot off the CD and follow the prompts. After that your computer should be clean.
Once your computer is up and running I would also download Malwarebytes antimalware from bleeping computers
Run it to check for any malware that AVG missed
Anti-virus dosent work