Well for OLD machines, i reccomend some small ubuntu distos, like DamnSmallLinux, or Puppy.
I was using Puppy and i was happy about it

Bout notSoOld but messy, well i recommend some older things, like LinuxSUSE, ArchLinux maybe too..
On almostNew i reccomend ubutnu maverick meercat 10.10 !

its have a preet nice gnome interface, very powerfull, very elastic,
On brand new eqiupment - for today (2011) i reccomend ubuntu 11.04 it is having some "mobile phone look like design" so its easiest, and have a new graphical engine caled 'unity'
u cann google and download all these systemz, its legal, and free,
but while u dont have windows u can have some device difficulities, for example i cannot print on my two printers becouse i was so unlocky to have two models that is not having linux drivers ;/ but for most of things u will have drivers, specially when its is popular.
Why i am talking about linux only? well, i was ..anoyed by bluescreens, viruses and PLEASE WAIT or ERRORs things on windows, so i kicked it out from my computers, and i am using linux. well after few years in linux i am happy

ps U can run most of programs and games if u have proper drivers and good parts, - from windows - at linux , just install in linux application "wine" , u cann google it with "hq wine download"
good luck.
(ps. u can run every distibution of linux live - from CD or DVD without installing, it will be slower - depending on ur dvdrom speed- but u can test if everything is working before u wonna switch to linux,
ps.2
u can also install windows and linux on same disk, remmember to install windows first, then install linux - a disk manager will take a free space on disk (u can change every single option) to create a linux partition in linux like ubuntu10.10 there is easy menu that will tell u what to do every single step. in ur language.
have fun .