Why not install with 2.4 and then use apt-get command to download/install 2.6 kernel? That is what I did. Piece of cake.
# apt-cache search kernel-image
It will then list what available and choose the latest kernel for your hardware (AMD K6, K7 or Intel i386, i686)
Use this as an example: kernel-image-2.6.8-1-686
# apt-get install kernel-image-2.6.8-1-686
# init 6
Login and make sure you have the latest version
# uname -r