hmmm on one of my machines, I am running Linux Mint 13 MATE edition, but then again. On my mini pc, I am running Linux Mint 12 L X D E edition. Cause There is NO L X D E edtion of Linux Mint 13. So I am good for now. the GUI/Window manager L X D E is actually a good one. Cause L X D E only uses 64 megabytes. But of course there Linux Distros specially made for VERY old hardware. Antix is one, But I did a quick check on distro watch
http://distrowatch.com/search.php?category=Old+Computers
GALPon MiniNo is another one, that aims for hardware that is 10 years old, or more. So keep digging.
Personally I started with Lubuntu (on my mini pc), and then I jumped a bit around, between Peppermint, Peppermint Two, Sabayon L X D E. Until I fell for Linux Mint L X D E for my mini pc, and there I first had Linux Mint 11 L X D E, and then now I have Linux Mint 12 L X D E. So the possibilities are basically endless.