Hi.
Puppy's worked grate for me on older machines as well, Fatdog64 even better (but not on the older, 64bit only...)
I have never had to boot USB yet but like having a spindles of old distros. If you have a CD drive that's what I'd do plus check-sum the ISO or hit DistroWatch
http://distrowatch.com/search.php
best wishes and have fun.