A few things to check (+ comments):
If the board is set for the CD first, I have seen some boards that requier you
to hit a key if a bootable CD is found, if you don't hit a key in a few seconds,
it'll go to the next drive. Watch for that message right after the post tests.
2nd, is your CD good? Are you sure? (retail or burn? double and triple check this)
In some odd BIOS's, I've seen where you had to select which HDD and/or CD you
want (like if there is 2 cd's, you have to specify WHICH cd you want to use).
It doesn't matter if HDD is primary/master and CD is secondary/slave - as long
as they are set up so they are not conflicting with eachother.