Start by looking at the wide number of Linux Live-CD's. These are bootable CD's that load Linux without using your HDD. See which Distro is most compatible with your hardware and which you like the feel of. Then you can install from the same Live-CD to your HDD, either as just Linux or a dual boot version of Windows and Linux.
My own favourite is PCLinux 2007. If you run XP then this Distro will look and feel very familiar. I use it a lot.
If you want something to run on an old PC then look at Slax which is a cut down version of Slackware. I can run this Distro on a 10 year old laptop with a 300MHz CPU and 4 Gb of HDD.
Others will recommend Ubuntu or Debian or ..... lots and lots available. So chose one that suits you.