In days where PC viruses are not that hard to remove, a free version on any anti virus product will fairly do the job well. But as hardware and software advances, so with this criminal who are developing viruses that is hard to detect, even with those paid versions of those popular anti virus software.
If you suspect that your PC is infected with a hard to detect unknown virus, and your not yet ready to reformat your hard drive, you can try the following method:
1. Remove your hard drive and full scan it on another virus free PC with updated virus definition.
2.Another method is to download a rescue disk on some popular anti virus product(Kaspersky, bitdefender, avira,Norton...etc,), you can google it and its free.
Method 2 is more comprehensive than method 1 but its more tedious.
The concept behind using a rescue disk is to create a bootable disk out of the downloaded image and use it to scan you system.In this case you can full scan your hard drive without booting at the infected system thereby preventing hard to detect viruses(rootkits) to load at startup.After downloading use Daemon tools or PowerISO software to create the bootable CD.
Take note, downloading and creating the rescue disk must be done on a clean and virus free PC.
I am using a Kaspersky Rescue Disk but other anti virus rescue disk will also do the thing.
Click here to download Kaspersky Rescue Disk