Thanks very much for your help everyone, but I have a story to tell.
Because I want to test my Hoster system at a karaoke I am doing tonight, I thought that I would take my system up to the local computer repair shop.
I told them the problem and asked of they could look at it really quick which they said they would. That was at 9.30am. They then phoned me at 3.30pm and said that they couldn't find anything wrong with it and that it was ready to be picked up.
So the moral of the story is, when your computer isn't doing what you want it to do, unplug all the leads and take it for a walk. You might look a little silly, but when you come back it should be working again.
Once again, thanks for the support.
Martyn