Google Docs can serve up PowerPoint (and PDF) documents in it’s document viewer. You don’t have to sign up for Google Docs, just upload it to your website, and call it from your page:
<iframe src="https://docs.google.com/gview?url=https://www.yourwebsite.com/powerpoint.ppt&embedded=true" style="width:600px; height:500px;" frameborder="0"></iframe>