Well my first tip is, are you using the right cd image of netinst install cd for your hardware?, i mean, x86, or amd64, etc.
Second tip, if you installed ubuntu successfully then i assume that your boot cd process is right, then, when you try to boot with your debian cd, your pc shows some kind of error or so? or just black screen?. Normally debian must show a menu right after the boot process with the cd has finished, in that menu debian let you choose among some options (graphic mode, text mode, framebuffer, etc) or to add some specific options to the install process, have you reached that menu?
If there is some kind of error just post it so we can workaround it.