Thanks for all the help. I was able to successfully replace my drive. I had to follow a slightly different procedure, though, so I thought I'd describe what I did for the benefit of anyone else who might find the need to replace one of their optical drives in this type of computer.
"Left" and "right" refer to the sides which would be in those positions when looking at the upright computer from the front.
1. Unplug the computer.
2. Lay the computer on its left side, so that the right side is now on top (I recommend doing this side first so that the computer does not have to be laid on its open cavity in step 4, below).
3. Remove the right panel. There are two tabs at the back of the panel which need to be pulled up (relative to the computer's current orientation) in order to release it. It's a little bit tricky, since I don't think Sony intended to have this panel removed by the consumer.
4. Either place the computer in an upright position or lay it on its right side, so that the left side is on top. The latter position may make removal of the panel slightly easier.
5. Remove the left panel. There are two tabs at the back of the panel which need to be pressed. These tabs are easy to release, since Sony designed this panel for removal by the consumer. Slide the panel towards the back slightly, then lift it away.
6. Return the computer to an upright position if it was laid on its side.
7. The top panel has a lip which covers the top portion of the front panel. The top panel must therefore be removed before the front panel can be removed. The panel is held in place by six plastic hooks or latches which extend from the bottom of the panel and snap into slots in the top of the internal metal case. These hooks must be pushed sideways before they can be pulled out of their slots. Some of the hooks are accessible only through small holes in the sheet metal case. In these cases, you will need to peer around with a flashlight and then, once you've located the hook, push it in using a long, thin instrument, such as a screwdriver. Some of them are a couple inches away from the "access holes," so it may take a little searching. Once all the hooks have been released, remove the top panel.
8. The front panel is held in place in the same fashion as the top panel -- with six plastic hooks which extend from the back of the panel and snap into slots in the front of the internal metal case. They are slightly easier to access than those of the top panel. Remove the front panel in the same manner as the top panel.
9. On the left side of the computer, unscrew the two screws holding the old drive in place. There are no screws on the right side -- they are held there by friction. Pull the drive out slightly to provide some working room. Detach the IDE cable and the power cable from the back of the drive. Pull the drive the rest of the way out.
10. Insert the new drive partway (remember to set the jumper properly, first). Attach the IDE and power cables. Insert the drive the rest of the way. Screw in one screw partway, leaving a little play on the drive, then screw in the second screw, and finally, finish securing the first screw.
11. Replace the front panel followed by the top panel. They should snap into place easily.
12. Snap the side panels back into place. This may be slightly easier to do if you put the computer on its side first. If you do it this way, I recommend replacing the left panel first, for the reason mentioned above.
13. Plug the computer back in and turn it on.
glad you have sorted it out 
