it seems your network card driver was compiled for your old kernel.
compile the new kernel again... but this time, un-install the old kernel source.
that way it will be impossible for drivers to be compiled for the wrong version.
also, have a look inside /usr/src/
you will probably find 2 kernel source folders, and old one and a new one.
ALSO, there will be an abreviated ( linux-2.4 ) symbolic link to one of the folders.
maybe your symbolic link was not updated to the new 2.4 kernel source when you up-graded, and the compiler followed it to the wrong source code.
thant all i can guess.
but delete the symbolic link, the slight convinience in an abreviated folder name does not make up for the havok it can cause when its wrong.