If you put "Ubuntu" into a search, you should easily find a version that you can download/burn directly to a bootable DVD (it may fit on a CD?) and you can try it out by booting from the disk. Of course you'd probaly want to put it on a hard drive (in a separate partition) if you decide to do much exploring of what it can do for you.

I believe I've heard of several others that are "portable" like Ubuntu, but was the one fairly generally suggested by users for an "introductory" look not too long ago.

Those actually using Linux probably can come up with links to what's best installed for more general use.

I used a bootable Ubuntu recently (downloaded within the past 2 months) when one of the kids showed up with an old clunker that a previous user had "cleaned out," that had NO operating system on it. The unknown previous owner's cleanout was a bit of a puzzle since some other stuff was left on the drive (i.e. he didn't just reformat) but the Ubuntu boot let me look at enough properties. Since it had only 1 MB RAM and a 30 MB hard drive (with similarly obsolete other hardware) it was easy to recommend it for the scrap heap.

John