This is easy in later versions, but you can get the effect in 2003.
- Use Autoshapes to draw a right triangle and rotate it and set it in the upper left corner.
- Set the Line color to No Line.
Now you just need for your corner effect to "arc":
- Use Autoshapes to draw a moon.
- Set Line color to No Line.
- Rotate and resize it to create the arc. The yellow diamond handle is a reshape handle- you can use it to control the amount of the arc. A good bit of the moon will be totally off the slide... don't worry about it, it cannot be seen during the slide show.
- When you have the effect you want, multi-select the shapes then right-click on one of them and choose Grouping... Group.