Thanks for that mgmcc, just to update this saga, I've since found out that the modem worked ok on the previous PC which had to be returned under warranty to PC World (the 2nd one since christmas) so this (3rd) PC which was given to them now has Vista on it. So I told him to phone ISP and ask for an up to date driver. I also managed to find a phone number for customer services at thompson for him to ring. Thompson said that they would send a vista driver to him for the princely sum of £2. I find it rather irritating that they couldn't have made it available as a download from their site. It's not as if Vista came along unannounced.
The NIS that came with the first PC is also for XP, so I've had to download the necessary and copy it onto the new PC for when he gets on the net to re-register.
My friend is on a very steep learning curve, as this was his first PC and he is not computer savvy at all, so this is all very discouraging as his first venture into the world of PCs and the WWW.
I will try to help as much as I can, and needless to say I have told him to visit this forum and to buy the magazine because you've all helped me with problems that I've faced.