Another distro worth considering is Puppy Linux. It's very compact, yet quite powerful and runs on a wide range of hardware, including older machines with limited RAM. It has various options for creating bootable USBs from a running Puppy installation. (Strangely, the Puppy sites do not offer a ready-to-go USB download: you need to get an ISO and boot from that, and then create a bootable USB. It is possible to create a bootable USB from the ISO directly if you know your way around Linux, but it's much easier to do it while running in Puppy).