Many machines have problems booting from a USB flash drive (even when the BIOS does have a boot from UBS option, which many older systems lack). So Hal's advice to carry the CD is likely the best advice. If you do insist on trying to use a flash drive to boot, it can be done with Knoppix on computers that properly support it. Puppy Linux and DSL are also good candidates, and one of these has utilities that configure the flash drive very cleanly (ok, I forget which one of these it is, but both are worth a look).