You should be able to move a Powerpoint file to your domain, then just point your browser to it: http://www.yourdomain.org/filename.ppt. The browser will ask whether you want to open it or save it. Remember, Powerpoint doesn't run on the server - it runs on whatever PC is opening the file.
Alternatively, you can put a link to it in an html file:
<a href="filename.ppt">Click here for the Powerpoint file</a>
Of course, if the person accessing the file doesn't have Powerpoint, or at least the freebies Powerpoint Viewer from MS, they're not going to able to do anything with the file. I don't believe there's a way for the browser itself to run the presentation, but perhaps someone else has a solution to that problem.
By the way, you can get the MS Powerpoint Viewer here (http://www.microsoft.com/downloads/details.aspx?FamilyID=428d5727-43ab-4f24-90b7-a94784af71a4&displaylang=en).
Edit: There seems to be something I wasn't aware of. According to this MS article (http://support.microsoft.com/?id=299824), a Powerpoint hyperlink on a webpage should launch the presentation in slide show mode in the browser. If that's true, that "a href=" syntax I provided should do the trick. I don't have Powerpoint, so I can't create a file to test it with!
Second Edit: I just played with it, and as far as I can tell, you have to have the Powerpoint Viewer on your PC, at a minimum, to be able to view the file.