If your interested in something to get you feet wet, try slax www.slax.org.
It can be put on a USB Flash drive and boot a computer.
I use Debian, Ubuntu and Fedora-11, but keep slax in the bag as a handy tool.
As they have all said, the best linux is the one that suits you best.
Get an external HDD so you can try many and not mess up the XP, any more than it is already.

I bookmarked Slax to future download.. That distro looks interesting

Well; as you can see, there are hundreds of different versions and flavours of the linux operating system. I've personally tried all the main contenders and found them wanting, too slow or resource-hungry like 'Doze etc. I keep coming back to Puppy linux. It's relatively easy to make the transition from Windows as there's a huge & very supportive online community. and once you've gained confidence you can re-master and produce your own version with apps, games to suit your own needs. Also, Puppy runs fast and happily from a live CD or USB stick, whereas all the others really want to be installed on the hard drive to get anywhere near matching Puppy's speed. If you have an XP computer I would recommend version 3.1, but if you're Vista and not FAT32 formatted I'd go with version 4.2. I've tried dozens of systems but found nothing that can match Puppy for versatility and speed.