When you boot with the Debian bootable CD, read the first screen which will tell you how to select the bf24 version kernel, which supports ext3.2 - HOW can I install Debian 3.0 with a 2.4 kernel and an ext3 filesystem?
BTW, having just (2 months ago) made the switch from Mandrake to Debian, don't expect the Debian install to automagically find all your hardware and appropriate modules as well as the Mandrake installer. A print out of your devices is useful. And suggest running either stable or testing vice unstable for a while. Unstable is correctly named.
Bob Scott