So here's what's been happening, and what I've done so far.
Mac has been lagging, running slowly and programs have been crashing. On Tuesday, everything froze, I shut it down, and it wouldn't start up again, just hung at the startup screen. After various trials and errors, I discovered a way to start up in a kind of DOS like screen (I think it's called single user mode, if that means anything to anyone) and was able to run something called fsck and then reboot, which got me going again.
So far, so good. But my research had told me that I needed to try a disk repair, running disk utilities. When I tried to do that, I was not able to, and it said that if I couldn't I should do it by running the OSX install disk and then go into disk utilities in there. When I did that, my HD didn't even show up, which my research now tells me means that my HD is in bad shape.
On the other hand, there are programs out there, that cost less than a new HD, that purport to fix things. And there's the possibility of an erase and reinstall. Any thoughts?
Meanwhile, I have at least been able to get Mac running again, though he seems fretful (I'm writing this on my old macbook, which is luckily just old and still runs perfectly well) and the other thing I've achieved is to format an external harddrive and backup my photos and all documents (which are really the only things I care about). Oh, and that reminds me, I should do my itunes library, too.
So I'm actually feeling quite pleased with myself. I know that many of you are much more accomplished computer whizzes than me, but I think I've just passed at least into the intermediate ranks :-)