The AVG Rescue CD is a bootable ISO image that comes with a range of tools to help you clean up a virus-infected PC, fix hard drive problems, and get an unbootable system working again.
Burn the disc to a CD, boot from it, and the integrated antivirus engine will attempt to update itself over the internet, or using a local folder. You can then run a full system scan to check for threats. And as any infection won't be active (because you've booted from the CD, not your PC), it won't be able to interfere or hide from the scan, and the chances of a successful cleanup are correspondingly much higher.
If your PC won't boot at all, then this doesn't necessarily mean you've been hit by malware. There could be another explanation, like a corrupt hard drive partition table. But the AVG Rescue CD can help here, too. Choose the Utilities menu, click TestDisk, and the program will check your PC for common boot and file system problems, and give you the option to fix anything it finds.
If that doesn't help, then you'll find a Registry Editor on the Utilities menu that could be useful. It'll let you access and change the Registry on the affected PC (assuming the partition is readable), which could let you, say, delete the references to a buggy boot driver that's preventing Windows from loading. But beware, don't play around with this unless you know exactly what you're doing, or you could make the problem even worse.
And if all else fails, then the Rescue CD comes with a file manager (Utilities > Midnight Commander) that will help you recover vital files from the stricken PC.