I prefer some of the newer distros to start on.
Instead of Slackware as a "run on bare metal" choice I'd go with Arch Linux if you really want to learn what it's all about on a very fundamental level. Good docs too.
Instead of Mandriva (who's future is still up in the air) I'd go with Linux Mint for an easy to use minimal hassle Linux that's very Windows like. A very polished and svelte distro, perfect for beginners - and more than powerful enough for an advanced user. Top user pick over at Distrowatch. And for good reason.
-40hz
Warning: Newbie hilarity ahead!

I finally quit being lazy and set out to try to see which distros behave on my older test box after getting skunked by Ubuntu, and Mint was first on my list. (Last year's entry was Open Suse which almost worked, so that's my counterweight.)
Unfortunately, Mint isn't behaving clean - it almost doesn't boot. I mashed a bunch of keys and stumbled into the command line, at which point I exhausted my fortune-cookie level of Linux knowledge by typing apt, then "went for the candy" with dist-upgrade, which at least was better in that it is now doing "something" (aka claiming to be installing all kinds of juicy packages. Before it wasn't even booting, so that's progress.) If that still doesn't work, I'll just have to try another distro.
Whew! Long story short, I'm not good enough to brute force a distro for the sake of the distro, for me as representative of a type of new linux user, having the distro be able to pass the test of "But Linux is good on older hardware!" is important. I don't really plan to do a full migration over like AC in the title post, I just wanted to know where the state of Linux is these days. In that sense I'll live with glitches, but boot problems are over my tolerance limit. This is just a random Best Buy box from a few years ago, maybe Pentium 4, so it should be right in the target market. With the advent of MS trying to push XP to end of life, I think we'll see a flood of former XP boxes floating around so Linux could be waiting in the wings for a nice small usage boost.
Edit: Still doesn't boot - gets stuck checking "battery state". So off I go to another distro...