I'm going to assume a little about your case here but......
In a typical case, you will have two sizes of slots. You will have the 3.5' size and the 5.25' size. The 3.5s are for floppy drives, zip drives, hard drives, etc. (basically anything that can fit into the slot). The 5.25 slots are for your CD-ROM, DVD-ROM type drives. The 3.5' devices, such as a standard hard drive will still be able to fit in a 5.25' slot but you will have to attach the metal braces that came with the drive. There are also casings that you can place the 3.5' drive into that will fit into the 5.25 slot.