Fedora Core 4
As of the date of this post there happens to be a new version of Fedora Core which would be number 4. This is my favorite Linux distribution. It is really easy to install especially on a PC you don't need anymore. Its packed with appliances and games. The only thing I have yet to fix how ever when you load this on an older machine and you use KDE or Gnome desktops it will run slower then I would like. But if you have never used Linux before this is the Linux distro I would recommend. I've heard that slackware is the best and gentoo is also good, except slackware is too pro oriented, and gentoo is getting up there. Anyway if you want to have fun with your old PC loading different distributions of Unix, Linux, BSD will become addictive and you'll learn allot too. Here's two tips: x86 is probably the ".iso" file you want, x86 being any pc that perhaps had windows on it, its the type of processor. An ".iso" file that you'll download is an CD image file, no not a picture but all the files that need to be burned to a CD. The way you do that is simply by selecting the iso file as if you were copying a CD in your CD burning software and instead of selecting a drive you select that iso file. One more thing that might sound daunting but if you stick the CD in and nothing happens try going into your bios and making your cdrom drive the master and your hard drive the slave then change it back when your done loading.
Get FedoraCore 4 here.
A good site for distros is distrowatch.com