Just to update everyone, and again big thanks to everyone who has tried to help.
This morning the computer wouldn't start at all, and gave us an error message saying the hard disk was about to fail, and that we needed to save all the data we wanted to keep.
Obviously we gave up trying to fix it and my wife took it to a small local business that specialise in computer repair. The guy seemed to think that the problem wasn't mechanic, but a virus, just a pretty f**king nasty one.
We're hoping he can fix it or at least save our photos and other files we wanted desperately to keep, fingers crossed it should be ok. If it is a virus, and he has to completely restore the system, we're looking at €90, which I thought wasn't too bad really, all things considered.
If it's mechanic, he'll get back to us (hope not as it would clearly be very expensive, but we have no choice if we don't want to lose our 18 months of photos).
I'll let you know how we get on, but big hugs because you're all a lovely lot!