"Avast Bart CD" is awesome
It has a program that you run. It downloads the latest updates, compiles the ISO and drops it in a folder of your choosing. then you just have to burn that ISO and boot it on the computer
It also includes scandisk (really useful, sometimes if windows won't boot this will fix it), a file explorer (so you can hook up a usb drive and recover files), registry cleaner, junk file cleaner, and a data shredder if you're donating a computer.
Many peoples use it regularly and the only problems encountes is on laptops that are preloaded with an encrypted hard drive that stores restoration information it, and a C drive partition stored within that. In those cases you cannot use this product.
The only bad thing is it costs money(subscription to updates), but is well worth it.Download it from here:
www.avast.com/eng/avast_bart_cd_downlo.html
or
You can use a bootable USB Stick. You could just update the virus definitions after booting from the key, and it would actually require even less work.
"ClamWin portable" is a portable AV scanner that will update on your USB