Puppy Linux Tahr 5.0.6 fits into 1G of disk space and runs well on only 500mb of RAM on an old XP box.
After playing with Puppy Linux, both the Slacko and Ubuntu 14.04 Trusty Tahr version, on CD, I decided to install Puppy Linux Tahr 6.0.5 as a dual boot alongside Win XP. The instructions online are not crystal clear to me, and left me somewhat puzzled, stomach ill at ease. Let me explain the process.
Puppy Linux can be used quite readily from your CD or USB stick. You need to boot up and then configure. I decided to dual boot install it on an old Win XP box, one with only 500mb ram.