Help much appreciated.
My PC will not go past the Welcome screen if any usb device is switched on. If I switch them off, or unplug them, boot continues.

What happens when you plug the device in again?

Is the USB device capable of booting, in which case you can demote it in the Boot list in BIOS.

No it can be a printer or scanner

So what happens when you plug the device in again?

Try reloading the USB root drivers.

Once I am past the welcome screen i can plug in any usb device and use them. I have uninstalled all USB controllers but same result.

Try disabling the Boot from USB device in the BIOS. It may be a quirk that makes it think your printer is a boot device, especially if it's one of those fancy jobs with a card reader type device installed (like the photosmart printers).

Have tried that, also disabled..`Boot from other devices``
I have not much hair left now.

Running out of suggestions.

Again, in the BIOS, have you got Legacy USB or USB Emulation enabled? if so, try disabling them. My guess is that it's in the BIOS somewhere.