Since your PC is already infected by viruses, the best and most secure way to remove them is to do a boot-time scan. What actually happens is that, when we boot/start our PC, a virus automatically starts itself in the background. In this case, the traditional ways of deleting a virus doesn't successfully delete the virus. The best way to remove a virus is to remove it before it executes itself. In boot time scan, we do the same thing. Any popular antivirus like AVG, Avast, Norton can do this. Here, we are using Avast! because it's free to use :
1. Firstly, download the Avast from its official site and install it on your PC.
2. Now, open the Dashboard by clicking on the tray icon of Avast.
3. Goto Scan tab and click on Boot-time scan.
4. Then, schedule a boot-time scan for the next startup. In the "Areas to Scan" option, choose "All Hard-Disks".
5. Now, restart your PC. It will scan your whole computer. Be sure, it will take longer time if you have tons of GBs on your PC.
Note : If it finds an infected file, it will ask you to Delete, Repair or Quarantine. I suggest you to choose "Quarantine" or "Move to Chest" here.