A typical PC can handle two pairs of "IDE devices" -- hard drives and CD or DVD drives. Two IDE sockets on the main board, one pair of drives each.
One of those is probably handling your CD/DVD drive already, and it's healthier if you don't try to mix a CD and a hard drive on the same cable. (You can usually get away with it, but if you're "unlucky" I don't want to be responsible for it.) Recent PCs will ship with a DVD and a CD burner on one cable, one hard drive on the other. If you only have one hard drive now, you're good to add one more, no problem.
If you already have two, it would be time to call in a geeky friend.