You Too! Friend of mine has also had (2) HDD`s. At least you`ve got the 4 CD`s though. I`ll look this up but if it`s the same as the other one the existing drive is "tatooed" to suit the installation sold to you. If you just want more disk space I strongly suggest you add the 120 Gb disk as a slave and keep data on that. The existing 20GB will serve as the Operating System disk. By far the simplest method.
If you buy a new HDD which you need to use as a master you need to get a new XP disc as well, your existing ones wont work on a new drive that isn`t tattooed. You could then transfer your data to the new drive from the 20GB and then wipe it to use as a slave 20gb storage disk. Unless anyone suggests differently.
If the current hard drive is working and you are only looking for additional storage space I would go with my first option. If you try to install the new drive as the master you would need to install ME from the original discs (I think) and then upgrade to XP from there. You would then need to slave your old disk and copy data.
Making the new drive the slave, if anything goes wrong again at least your data is intact on the newer drive.