How do I make an installation initrd.img for a new kernel?

[Barry Scott <barry scott onelan co uk>]

I need up update the network install for FC4 so that it
supports new hardware (nVidia ethernet - forcedeth).

The 2.6.17 kernel is fine in this case. But the initrd.gz that
is installed in /boot is nothing like the images/pxeboot/initrd.img
from the installation CD.

I need to know how the images/pxeboot/initrd.img  was made
so that I can make a new one for the 2.6.17 kernel.

Can anyone point in the right direction?

Barry