In case your PC is heavily infected with viruses it doesnt make sense to try to clean it after installing antivirus software. This is because the virus may be capable of messing with the antivirus installation. In this case, the solution is to use a Rescue Disk system. A rescue disk is a bootable disk, i.e it does not require booting off the hard disk. Hence viruses in the hard disk boot sector are not loaded to memory.

The usual way to create a rescue system is to create a CD or DVD based rescue disk. But in situations where you don’t have access to a writable CD or DVD, or if your laptop or netbook doesn’t have a working CD drive, you may create a rescue disk with a USB. I’ll explain the steps for creating a rescue system using Avira Antivirus and a flash usb drive.

On a clean PC/Laptop, install Avira Antivirus.
Open Avira Antivirus.
Extras > Download rescue CD > Start

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When the download is done, the software prompts you to insert a CD. Ignore it and click Cancel.
Now you will be asked if you want to save an iso. Click Yes, and save the iso file.
Download Rufus, a software to create bootable usb disks.
Open Rufus, and select “Create a bootable disk using” option to select the iso you saved.

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Click Start.
Write protect the USB disk.

Shutdown the infected laptop/pc.
Now, insert the usb disk into it.
Boot the pc.
Avira rescue system, based on Ubuntu opens. Allow it to update virus definitions, and scan all the drives. It will clean the detected viruses.