There is a BIOS setting to determine the boot sequence. Generally it
should be set to floppy, then HDD. It can be set to try the CDROM
before it goes to the hard disk, which I uspect is the case.
Sometimes it is phrased like "Boot Device 1: IDE0, Boot Device 2:
IDE1" etc. These can be tried, trial and error, until you have a
successful boot without that message. Just be sure to write down the
original settings before you change them in case you mess something
up, nothing personal. As long as you can get into the BIOS this should
be easy to solve.