Well if you have powerpoint itself you should be able to got the slide you want and save it from there.
If you are using the free powerpoint viewer I think the only why is if you press the "Print Screen" button on you keyboard (assuming a pc) and then run something like paint (or better yet get a demo copy of Paint Shop Pro
http://www.jasc.com/ ) and goto the edit menu and click paste.
I hope that is clear and helps, let me know either way.