Is there a free anti-virus program that can run directly from a cd without being installed on the computer?

I do computer work and i need something that I can check customers PCs with, without installing anything on it. I know years ago, Norton Antivirus could do this, but I stay away from Norton now, it sucks.

I also know that the definitions need to be updated, but what I'll do is download the latest files and put them on the CD the day i go to a customer.

Also, what about putting it on a thumb drive? Is there a program that can install itself on one of those. That would be even better.