Subject to further testing, I believe I have got this solved. But the whole operation has raised a lot more questions.
My laptop has a Turion 64 processor and ATI chipset, and is an Acer 5536. Mysteriously, during the months I have been looking at this, a usb driver for the machine appeared in the Win7 64-bit driver section of the Acer support site. It was quite old and ominously had the letters XP in the file name. Acer seem to have at least 3 different driver sites and it looks to me as though someone is trying to make them all contain the same files.
Where I say install, this always involves a subsequent reboot.
I installed it, but then couldn't find any reference to it anywhere on the machine, and the appalling usb1.1 interface performance remained the same.
Following a really helpful post from someone who had found a cure, I went, on his advice, to
http://sites.amd.com/us/game/downloads/Pages/integrated_win7-64.aspx#3
and downloaded the current ver 11.2 South Bridge driver. This comes with a "Catalyst" installer which crashes on my machine, but only after creating an ATI directory with all the files in it. Drill down through Packages, Drivers, SB8xx and you get to a USB Filter directory, with a pdf file that referred to messages I was seeing on the glitching usb bus. I followed their installation procedure as per the pdf, but saw none of the messages that they said to expect. However, my usb 1.1 audio interfaces now work fine.
The "Catalyst" appcrash points to ntdll.dll, a Microsoft file. I'm now trying to work out why my machine has 2 of these in different directories and of different sizes, but both with the same description details. Son with Win 7 Ultimate has 2 different different sizes.
Sorry this has taken so long. Hope it helps someone else.