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Roby71 - given you've tried all the obvious things, and that you have probably read & considered what's in the the
FAQ section of the manual, I suspect its another program that you run on that specific PC - could be that you don't even know its there.
If you have Process Explorer (available from MS for free) then you could save a list of running processes from the different machines, then you could see if there's something running the the problem XP box and not the others.
Or you could close down all the 3rd party programs that are running including those in the tray - msi config lists what it starts at boot time in its Startup tab. If calibre metadata download is then OK, then restart them one by one until it stops working. Then decide what to do with the other program - ditch it, stop it when using calibre etc
BR