There will be a jumper on the back of the hard drive (a little peice of plastic) which will be over some pins. there will be CS (cable select), MA (master) and SL (slave), or something similar.
If you pull the hard drive out you will see a sticker that should explain where to put the jumper.
If your original hard drive is set to master make this one set to slave. if your original hard drive is set to cable select make this one set to cable select. Just remember that primary drive is on the last connector on the ide cable and the drive you just got goes on the other one(middle one so to speak).