There is a good howto at [http://kerneltrap.org/node/view/799
First make sure you install 'module-init-tools' if you don't already have 2.6 [do a 'uname -r' to check], read the README and INSTALL notes. There is an option for keeping to old tools so that your system can run 2.4 and 2.6, you also need to give the proper prefix ['./configure --prefix=/', I should think.].
Obviously don't overwrite your working kernel with one you don't know works! Add an extra option to grub or lilo.