|"I got a message that the reply would be saved once I'd logged in - but no, it's gone"|
It wasn't gone. If you would have refreshed the page after logging in, your reply would have been posted. If you didn't do that, then you lost it.
"It won't boot in Safe mode - I just get to see a long list of partitions and drivers"
Seeing "the list" is normal. You probably didn't wait long enough for safe mode to kick in after the list was displayed. Sometimes it takes several minutes.
"Now I did burn it to the CD but perhaps it wasn't an ISO image burn? I've really no idea"
An ISO file has to be "unpacked" with burning software that supports image files, such as IMGBURN (as suggested by Derek). If you simply copied the ISO file to the disk, that's why it does nothing.
"Just to correct phil22 in that you can't use an sp2 disc to repair xp with sp3"
And I agree with trek1701A, you can use a SP2 disc to repair an SP3 installation, however, you will lose SP3 & any other MS Updates that may have been installed. You would simply have to reinstall them all after the repair is completed.
"I have ordered a disk caddy to see if I can remove the HD and access it from my laptop"
Totally unnecessary. Either access your files using a properly burned LINUX live CD (IMO, Ubuntu is a poor choice for that), or simply do a repair-installation using the SP2 disc. Of course, if you have a hardware problem, all of this is a complete waste of time.