Is it a new build or an existing one that just suddenly started exhibiting this problem?
Unplug both and confirm it will boot. Then plug in either the cd or HDD to see if it is one or the other causing the problem.
Check cabling (IDE) and the power connector to make sure both are firmly attached.
If it's a new build check jumpering (ex. WD set as master on a channel by itself usually needs the jumpers removed to work properely, or make sure you're not mixing cable select and regular jumpering).
Fern