My advice is to restore the better computer for Windows.
then you try installing Debian, ubuntu, xubuntu or slackware to one of the older systems, avoid Knoppix for that as both above have stated, it is not made for hdd, play around som

Debian and Slackware are slightly harder than Ubuntu and xubuntu to install but all are linux at heart.

http://www.ubuntu.com/ (fairly easy)
http://www.xubuntu.org/ (more or less same as above but lighter load on system)
http://www.debian.org/ (where ubuntu, Knoppix, DSL, Xubuntu and many others started out)
http://www.slackware.com/ (one of the two oldest Linux distributions)