First get the root shell by entering the following at the boot prompt:
boot: Linux vga=normal s
Here, Linux is the label for the kernel image you are booting; "vga=normal" will make sure lilo runs in normal VGA screen, and "s" (or "S") is the parameter passed to init to invoke single-user mode. Enter the root password at the prompt.
There are few ways to disable all the X starting daemons:
run update-rc.d -f ?dm remove ; update-rc.d ?dm stop 99 1 2 3 4 5 6 .
run update-rc.d -f gdm remove ; update-rc.d gdm stop 99 1 2 3 4 5 6 .
insert "exit 0" at the start of all /etc/init.d/?dm files
For gnome edit /etc/init.d/gdm file
add the following line in the first line
Save and exit the file
rename all /etc/rc2.d/S99?dm files to /etc/rc2.d/K99?dm
mv /etc/rc2.d/S99gdm /etc/rc2.d/K99gdm (may be more file)
remove all /etc/rc2.d/S99?dm files
Here, number in rc2.d must correspond to the runlevel specified in the /etc/inittab. Also ?dm means that you need to run the command multiple times by substituting it with all of the xdm, gdm, kdm, and wdm.
Only the first one in the list is "the one true way" in Debian. The last one is easy but only works on Debian and requires you to set the display manager again later using dpkg-reconfigure. Others are generic methods to disable daemons.
You can still start X with the startx command from any console shell.