Powerpoint will automatically display on the computer in the format that a presentation was created. No setting required.
You can set Powerpoint to create slides in either 16:9 (which is now the default) or 4x3. You can convert from one to the other, but this tends to mess up the layout of slides a bit.
In my experience any issues with format tend to come from projector settings... are there settings on the projector to change the format? Usually a 4:3 projector will show 16.9 slides letterboxed.
Hope that may be some help...