Using old machines not PXE capable
Yes, computeMode can be used to create a cluster even with quite old machines.
However, those machines often do not provide the PXE boot method.
Then just write the floppy image on a floppy disk, insert the disk into the drive and change to boot order to first try the floppy.
You are all set.
Rq: It is also possible to use a PXE boot from CD or from a USB key for instance.
PXE on a disk
You may also have a look atthe pxe-on-a-disk page, that provides a PXE disk that should boot using any network card by detecting the actual driver to use.