When double-clicking on an Outlook Express email message to open the message in a new window, the window does not appear full screen so I must always click the ‘Maximise’ button. How can I make the window open maximised by default?

You cannot set Outlook Express to open the messages in the maximised state, but instead you can resize the window to take up the full screen, essentially mimicking a maximised state. Open Outlook Express and then double-click a message, to open the message in a new window. Make sure the window is ‘restored’ size (i.e. not maximised or minimised, just the normal fluid size which you can adjust with the mouse). Then, grab the sides of the window and manually resize the window to your desired dimensions. In your case, you would want to resize the window to the full screen size. Once the window is resized to your desired size, click the X button to close the window. Then, double-click the message again (or another message) and the window should open in the size you previously configured (e.g. full screen size).