Thanks for the info, I'm not actually sure what I've got. XP came pre-installed on this computer and a disk did come with it but I can't for the life of me find it at the moment.
I also have an old XP upgrade disk but I don't believe i can use that for for booting.

Anyhow, it doesn't really matter now. I did my usual frustrated rant on a forum and when I calmed down I decided to give it "one more try" at sorting the modem out. I did the same thing as I had done before when it didn't work and before that when it did and it actually installed properly again and I'm back on the net.
There's still a huge amount I don't understand about Linux but I keep trying.