Well there are really two types of drive that are in use, either IDE or SATA, you will need to take the side off of to find out which.
So this maybe a bit of a long route to get the info but i havent used XP in a while.
So click START - then CONTROL PANEL - then SYSTEM - then HARDWARE - then DEVICE MANAGER
look for DVD/CD it should be about 4 down, double click, then on the new window click DETAILS, it should say IDE or SATA.
an LG DVD that is SATA
http://www.aria.co.uk/Products/Comp...ATA+black+bare+inc+SecurDisc+?productId=30468
An LG DVD that is IDE
http://www.aria.co.uk/SuperSpecials/Other+products/LG+GH22NP20+22x+DVDRW+Black+OEM?productId=33832
both will read/write CD and DVD, will simply be an issue of opening the PC case, removing two cables from the back of the DVD drive you have, inserting new DVD drive and reattaching cables.
Or you will have to open the side of the case unplug it, then remove the dvd from the front of the case or even pull the front of the case off, but its all pretty easy and we can tackle that when we get to it.
Hope this has helped and any more questions please feel free to ask, everyone is real freindly and helpful here, if not a bit sarcastic sometimes