Thanks Mick - stuffing the Sunday Chicken
I can't find the links on this forum.
The long description is HERE but in a nutshell, repeated errors can cause windows to default to Programmed Input Output mode, the very slowest of all. It then never resets itself. I'd recoment the Uninstalling the Port method describe some way down. Your hard disc should then get good rates. If it starts to slow down again, then it must be on the edge of a failure and I'd suggest a replacement before you lose all your data.