basics for a second hard drive is they are configured as slave and master ,there will be jumpers you can set up on the back of the drives( Csel ,slave ,master ) .Since you are adding one it will be set for master out of the box and the one you have in the computer will be recognized as master(if its Cable Select change it to master ) . You will change the one you add to slave .
With the IDE ribbon (cable) the connector furthest away from the from the motherboard connector will be for the master drive the one in the middle will be the slave .Shut down the computer, before you add the drive make sure that the connectors are properly seated ,dont force anything and then restart it .Windows XP will recognize the new hardware