It should be
vmlinuz-2.4.18-bf2.4 at least that's what it was on my woody firewall box.
Code:
stephen@SleepyTux:~$ ls -l /boot/vmlinuz*
lrwxrwxrwx 1 root root 20 Jul 17 2003 /boot/vmlinuz -> vmlinuz-2.4.18-bf2.4
-rw-r--r-- 1 root root 1263339 Apr 22 2003 /boot/vmlinuz-2.4.18-bf2.4
-rw-r--r-- 1 root root 1017191 Dec 3 06:45 /boot/vmlinuz-2.4.23.031203-st
-rw-r--r-- 1 root root 1017092 Jan 8 09:41 /boot/vmlinuz-2.4.24.040108-st
Edit: You do know that you are expected to upgrade the install kernel with either a self compile or via apt-get the packaging system does not know about the install kernel (there are security problems with the kernel you get installed from the disk) so you will never be notified of the problem when using apt.