I installed the latest Ubuntu version of Linux on one of my computers that was already running XP Pro+SP3. It did a beautiful job carving out the necessary space on one of the hard drives and setting up a dual boot scenario so I can boot XP or Ubuntu. This is a great way to learn to use Linux while still having your trusty XP available when needed.
I still prefer XP but the Ubuntu flavor of Linux is not bad.
Linux will no longer install on older, slower, low memory system, at least not the flavors I've tried. I was thinking of installing it on some old computers we got donated but that turned out not to work. I tried Ubuntu and Knopix and one other whose name I forget at the moment and they all wanted a minimum of 256MB of ram.
Puppy Linux might install on these system, have not tried it (yet).
Can I just reinforce Rich's comments.
Ubuntu in its latest distro is a very powerful and well thought through OS
Whereas in the past Linux used to be regarded (by some) as the preserve of the "experienced" computer/owner/operator, a lot of folks now feel that the Ubunto distro (among others) has been refined and simplified to the extent that a lot of folks are taking a serious look at it.
Try it and see what you think
DSL has a tiny "footprint" and I even have another version that can be run from a floppy disc, with no hard drive involvement and with internet access…… but I don't try to use it everyday!
Regards
paws