Unix without a GUI for old machine

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http://unix.stackexchange.com – I have been given my father's old computer, which currently runs Windows 95 and has a 133-MHz processor, a Pentium(r) according to "My Computer". It seems a shame to recycle it when it works so well (considering what it is), so I'd like to load some variant of Unix on it and learn some stuff. I happen to have a copy of "Unix for Dummies" 4th edition (copyright 1998), but it assumes someone has already set up Unix on the computer in question, so I need another source for this step. There are several existing questions on here about the Unix-on-old-computer thing (typically Linux), except for o (HowTos)