What BIOS version is your Dell running? I don't seem to have any problems with the wife's machine.
I turn off the CD in the boot options.......set the PXE BIS Default Policy to off.
Then when I use the F12 option and select CD, I get a Device Not Found message.
It offers an option to retry which continuously produces the same message and an F2 option to reset the BIOS. I guess if you password protect the BIOS setting then that isn't an issue?