There's any number of great antivirus tools that help protect your PC from viruses, but what about when you encounter an already-infected PC? Your best bet is a boot CD, and the free AVG Rescue CD cleans viruses easily. The AVG Rescue CD comes in two flavors: an ISO image that can be easily burned to an optical disc, or a compressed version that can be installed to a bootable flash drive. Once you've done so, you can simply boot from the drive of choice directly to the AVG menu, where you can scan for viruses, edit files, test your drive, or even edit the registry. Since the bootable CD is based on a version of Linux, you can also access a number of common Linux tools to make changes to your system and hopefully make it bootable again.