Thanks!
A shame @heinogandadoesn't seem to be around at the moment, as you say.
I have never managed to get System Restore to work on my main desktop machine, it always goes through the motions, and then says that the system couldn't be restored, no reason given, and to try another restore point. Needless to say the same thing happens with all of them!
I do have a simple file backup which I do every week, but not from before when the problem appeared I'm afraid.
I've now found that the same problems are happening on my XP netbook too, so it looks like it's a wider issue.