Open up your case - Hopefully yours isn't as messy as mine is from the SCSI cable being 10X longer than it needs to be plus all of the power Y splitters going to all of the fans.

Now you should be able to find an open 3.5" slot somewhere. I have enough space to mount 6 in mine. The bottom rack thing is where my 15k SCSI drive is. I have a pair of case fans to the right of that keeping good airflow over my fast hard drive. Above that I have my storage drive which is IDE.

Slide the drive into an available slot and find 2 or 4 case screws. Two if you're lazy like me and only screw things in on the side that I get into. Four if you never touch anything in your system. For that you will have to pull off both side panels to your case. The case screws are bigger than the ones used to screw in your CD drives.

After the drive is secured then go ahead and connect the power and IDE cables. On the edge of one of the cables you will see a line. Normally it's red on the grey cables and white on black cables etc there will be some kind of colored line to indicate pin 1. This pin always goes on the same side as the power connector.

To connect the IDE cable to the motherboard you'll have to find something that looks like this:

Each connector represents 1 IDE channel. Most boards have 2 channels while 4 is becoming more and more common at least on the higher end boards.

Each channel can have 2 devices on it.