Question Author
Thanks Mamya. Unfortunately the video clip just shows how to align the icons which I know how to do, but it doesn't say how to keep them like that.
It's never happened before, the icons have always stayed where I place them, but for some strange reason they have become all mixed up and spread out all over the desktop 2 days in a row.
I've arranged them back again and will just have to hope that I can find out how to keep them in that order before I switch off my computer tonight.
If they are all mixed up again tomorrow, I might just have to open the window and throw the pesky thing out!