Recently, both Adobe Reader and Adobe Acrobat have failed to display PDF pages within Explorer, opening their own window instead. The option "Display PDF in browser", in the Adobe Preferences dialogue, is ticked, but it makes no difference. Adobe Reader displayed a message once to the effect that the cause was a configuration problem and would I like it fixed, to which I answered yes, but it made no difference and now the message doesn't appear. I'm not aware of any change in configuration in my system that could have caused the problem. Does anyone know what's going on?