Configure the jumper on the new HDD to slave (I can tell you how if you need me to, just need a model#). Mount it into the case, connect the IDE cable and boot up. You will need to format the new drive my either entering CMOS and entering the commands, or if you have XP you can use the built in Disk Management tool to do it.
Any more questions or if any of this is unclear, feel free to ask.
System specs? Lets just get down to it people:
My penis is bigger than yours.