Kaspersky Rescue Disk 10 is a free bootable environment that will detect and remove malware from a badly infected PC.
The system comes as an ISO image. Burn this to a DVD and you should be able to boot your PC from it. If your system is already infected with malware then this makes it far more likely that you'll be able to remove it successfully, as the malware won't be running and so cannot apply any tricks to hide from the scan, or prevent itself from being deleted.
Of course the problem with this approach is that a DVD will always start with the same virus database. Fortunately Kaspersky Rescue Disk 10 will detect and recognise any network connection, so if you're connected to a router then it's able to go online and download the latest virus definitions.
After that the system works much like any other antivirus tool. Click the Objects Scan tab, choose to scan your PC, and Kaspersky Rescue Disk 10 will crawl over your hard drive looking for malware and removing anything it finds.
Please note, if you'd prefer to install Kaspersky Rescue Disk 10 to a USB flash drive, rather than a DVD, then that may possible. With a little work, at least. Check this Kaspersky support page for the instructions you need.
Every PC owner needs a disk or two like this, just in case their regular antivirus tool is caught out by a brand new threat. Go download it immediately