Having just moved from Mac to PC there are some annoyances we've found and the Windows Open/Save Dialog being off screen and unable to move is one of them. Luckily there is a simple fix:-
1. Press Alt+Space
2. Press Move
3. Use your arrow-keys / Move your mouse or stylus around.
You will now be able to see the full dialog box once re-positioned and then you can use/resize it as normal