I have the same problem with my laptop (Compaq Presario). I'm still working at
it but I know somes things that can help you. First, somes says than ACPI support is broken under Mandrake 9.1, so 9.2 might be too.
I found interesting sites about peoples who successfully solved the problem, go to http://www.stud.ntnu.no/~fagerlun/evo_n800c/
for an example. He said he downloaded the latest ACPI Patch for his kernel at http://sourceforge.net/projects/acpi/
and then recompiled it with ACPI support. It worked for him but not for me, I have trouble with my configuration. Maybe you'll have to modify the /etc/lilo.conf file to enable ACPI support (by default, I think it's set to ACPI=ht).
Right now, that's all. If you find something about it, let me know.
P.S. I'm speaking french, I hope my message is comprehensible.