If you are rather familliar to Unix, I strongly recomend Slackware 9.0 since slack is the most unix-like distro out there. It seems a little scary to install but is not really that bad. It is also *very* good on lower spec machines- one of my favorites.
If you want something *really* easy to install, use, and configure, try Mandrake 9.1, or SuSe. They are two of the *easiest* distros to use - probably rivaling XP in many ways. They are, however, a little harsh on lower speed machines and you can not get SuSE as a free ISO download (you can get it free via FTP install, but I have never figured out how to get it to work).
Go to http://www.linuxiso.org
, this site has many Linux ISO downloads available (for free) as well as general discriptions and links to the Distro's homepage..