Having just updated all three systems to Beta #8, the two desktops now both work fine in debug, but the laptop still doesn't work. It is as if it is something to do with signatures on the files that the AV is detecting, but as I'm using different versions of windows, different AV software and it still fails with the AV and Firewalls turned off, I can't explain it.
Now my two desktops are working fine with debug, I'm not too worried about the laptop, but I do hate those unsolved mysteries.