computer powerpoint

Easily.
Just unplug the projector or switch it off.
You could always just put something in the way of the projector too.
I don't know if there is a way to do this in software but I suspect not.

I think you misunderstood the question. It seems that the poster wants to have the slides and notes show on the laptop and only the slides on the screen (without the notes)

I am not aware of any way to do this since the projected view is a copy of the screen view.

Ahhh yes. I see what you mean.
Well.
There's this fantastic new invention I've been reading about on one of the new technology gadget websites.
What it allows you to do is look at any information from pretty much any program either while running the program with other data or actually not running the program at all.
So you could say have a presentation running with the main display while also looking at the other data you want without that other data being visbile to the people watching the main display on the projector.
It will work with almost anything apart from games but if they will allow a pause for a screenshot it can even be used for those too.
The data you can look at in privacy isn't limited to graphics or to just text either. You can have combinations of the two and it also allows you to review the data at any time you like during the presentation.
If the people viewing the presentation have their own viewers they can even look at the material too if you want them too.
All without disturbing the look of the primary display on the projector.

Brilliant huh ?

By the way....

It's called "paper".
Weird name but I'm not a marketing man.
They think it's going to take off.