Thank you for the reply. You are exactly right, I did a make modules_install after I did make modules.
I'm afraid I'm enough of a novice that I don't understand how to fix the problem. I take it that now that I have different /lib/modules directories, I have to do the latter of your suggestions, boot the the new kernel and install ther kernel module from so that it's in /lib/modules/2.4.blah. I obviously don't understand, because this is what I thought I did, by rerunning the nvidia package in the new kernel. This is what caused X on the old kernel to stop working and also, I guess, prevented me from rerunning the nvidia package in the old kernel. How do I restore X on that kernel?
Also, what *should* I have done? Not done 'make modules_install' if the kernels are builds of the same version?