SOLUTION SOLUTION SOLUTION SOLUTION
After three days work I had given up and repacked the DWA-131 to return it.
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I think that the problem was as soon as I inserted the adapter the PC (running Vista) found an existing driver that it thought was the best option somewhere on the hard drive. Disabling auto install had no effect. Having looked for it and deleted anything that looked like an old version of the driver it still managed to find a version of the driver that the DWA-131 happily ignored. I had tried everything.
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Having not touched it for a day or so it occurred to me that I might try installing the driver in Safe Mode. As a last stab I uninstalled the software, rebooted in Safe Mode (basic option with no internet drivers either) and ran the install software from the CD supplied. It went through the installation and when asked to insert the hardware it found the driver. It would not run the connect phase of the software. Rebooted the PC as a normal startup, found the installed software, and when run it found the network that had been missing for so long.
Hit the WPS button on the router and bingo! WIFI WORKS