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You probably have a configuration file in /boot that corresponds to your current working kernel - the file should be called something like /boot/config-2.6.7-K7-4 or something along those lines. cd
to the directory containing the kernel version you want to configure and type cp /boot/config-2.6.7-K7-4 ./.config
. After that you should be able to run make oldconfig
(to answer yes/no/module for all new options that wasn't in the old kernel) or make menuconfig
(where you'll see new options marked as "New").
With 2.6 kernels it is also possible that the kernel config is stored as /proc/config.gz. That would work instead of the config file found in /boot as well.
If you run Debian you should seriously look into using kernel-package to build and package your kernels. It makes life much easier. There are several tutorials on using it on the web.