It's quite easy - open the presentation in Keynote, click on file, and down the list you will see "EXPORT". Click on that, it will give you an option, choose from .ppt from the list and presto, the Keynote file will appear in your documents file with 2 files, one will be Keynote and one will be .ppt.
While you are at it, why don't you click on "send to" from the file menu, so you can send the presentation to your email account with 1 click.
While PP will play Keynote presentations, it can't do all the transitions etc. that Keynote can. It will then downgrade them to something it can do so your presentation may end up looking more basic.