Backup all the importnat stuff just incase something goes really really wrong.
Turn of all the power and disconnect everything. Open up that base unit.
Locate the hard drive. Now (with luck) There'll be a 40 or 80 wire IDE cable with 3 connections. 2 at either ends one in the middle.
Connect the new drive to the one in the middle.
Also on the back on the drive there'll be "jumpers" a little fiddly thing. Pop it off and repostition it. Only do this for the drive you are installing. On the drive itself it should tell you where it goes. You have to set to "SLAVE". Look on the shiny side of the drive.
You will also need an extra power connector.
So just make sure you connect it to a power connector, install the IDE cable and repostion the jumper.
Turn on the machine and hopefully it will Autodetect the drive and you will not have to do anything and then you can acces the drive from XP.
Oh and make you secure the drive with the screws.
(i hope there is room)