<p>If you're thinking of carefully balancing your projector on it's side, then some Windows graphics drivers allow you to rotate the display in their advanced drivers. The keyboard shortcut is sometimes ctrl+alt+a_cursor_key but success may very depending on the manufacturer of the display adapter.</p>
<p>OpenLP should then (in theory) detect the new resolution and adapt next time you run it. I've not tried it though...</p>
<p>Note however that videos may not work so well!</p>