This is a very interesting problem. Just to give you some insight as to what is happening here may help you understand what is going on;
When a new kernel is installed the modules defined in the kernel are created in /lib/modules/<kernel_number> and modconf is pointed to that module.
As I understand the problem you have installed kernels 2.2.20-idepci and 2.4.18 but modconf is for some reason pointing to 2.4.20. Since you have not installed that kernel the modules don't exist in /lib/modules/2.4.20.
You have 2 ways to resolve the problem;
1) do a new clean install of bf24 or;
2) download the Debian 2.4.20 kernel relevant to your system and type here
to /usr/src and install by doing cd /usr/src/ and then dpkg -i kernel-image-2.4.20*.deb
Hope this helps.