Few of the commonly available PCs come in anything but mini-tower boxes, and many are micro-tower. Very few of them have an available bay for any additional drives, or for a drive enclosure, if you get the common CD/DCD-ROM, CD/DVD-R/RW, and a floppy or zip drive.
There is often an internal space, often with integral rails, for installing an additional hard drive, but a configuration with an externally accessible bay is very rare. Few configurations make the second internal hard drive really usable, since the common accessories use up all the outputs from the EIDE controller, so you nearly always have to add a card to add a hard drive - or much of anything else.
You almost have to build your own to choose a chassis that has an extra bay.