What Is The Best Linux Distro For An Old PC?

I thoroughly recommend Linux Mint 9 LTS which is built upon Ubuntu 10.04 LTS its easy to install and easy to use plus it comes with much of the software you are likely to need preinstalled

Linux Mint 9 LTS Download
http://www.linuxmint.com/edition.php?id=52

Linux Mint 9 LTS Installation Guide
http://www.linuxbsdos.com/2009/12/03/linux-mint-8-installation-guide/

Linux Mint 9 LTS Release Notes/User Guide
http://www.linuxmint.com/rel_isadora.php

You download the ISO. image of Linux Mint 9 LTS then you need to create a Bootable LiveCD for installation
https://help.ubuntu.com/community/BurningIsoHowto

Linux Mint 9 LTS can also be run direct from the LiveCD from Boot without touching your Hard Drive

Once you have installed Linux Mint 9 LTS you can bring it up to date with easy tweaks like upgrading the Kernel to 2.6.38 and installing Firefox 4

Upgrade to Kernel 2.6.38 on Ubuntu 10.04 LTS/Linux Mint 9 LTS
https://sites.google.com/site/lightrush/random-1/howtoinstalllinuxkernel2638onubuntu1004lucidfromubuntu1104nattytheeasyway

How to install Firefox 4 in Ubuntu 10.04 LTS/Ubuntu 10.10 from the Firefox PPA (stable)
http://www.webupd8.org/2011/03/install-firefox-4-in-ubuntu-1004-1010.html

LUg.

Thanks

Linux Format and Linux Magazine (out of the UK, not the US mag by the same name) both provide DVDs that mean that criteria.

You can get Ubuntu, Mandriva, Suse, and other such distros like that. Just avoid multi-distro DVDs which have smaller, less complete versions since you won’t be able to get online to add or upgrade packages.