I've now downloaded clusterKnoppix, via gFTP (Konqueror didn't work), and the iso MD5 checksum was correct. But when I boot the CD I get a bunch of errors (loop.o not found, etc.). And when setting up the PXE server I get the message that my network card is not configured, no matter what I do next it won't start the server. And the network card indeed doesn't work.

Testcd tells me that there are bad md5 sums, so it's probably the (fresh) 1/2/4x CD-RW medium that gives errors. I've reburned it twice (same CD), and I'm now burning it over my older copy of 'vanilla' Knoppix at 2x, so I hope that (used) CD-RW will work.