FrozenTech has made a little test of boot time for different popular distributions – the winners are Damn Small Linux and Knoppix on the older machine.
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For the old machine:
Damn Small Linux booted much quicker than all other distros in this test. It was also suited well after booting, with all of its apps being small, it did not bog down the laptop. While Knoppix and SLAX were able to load, many of the apps included with them took a long, long time to load
For the newer (faster, more RAM) machine:
SLAX was the quickest booting in these tests, into both Fluxbox and KDE. All the KDE and Gnome LiveCDs that could not boot on that laptop had no problems with a full gig of ram to work with.