Xubuntu is worth a shot too. XFCE is pretty light.
Getting Linux really, really light can be done if you dont mind spending a night or two on it. Look at Arch and Gentoo.
If it's an XP laptop I second XFCE. My choice is Linux Mint XFCE though, as it's much nicer on the eyes and you could be forgiven for mistaking it for MATE if you don't look too closely. It's also mega light.
Failing that try something like ArchBang which has a super fast OpenBox DE and has auto-updating menus unlike CrunchBang.