This really isn't a Typography Question as much as an Acrobat Question (assuming you are using Acrobat). In some versions of the Windows OS there is a bug printing to postscript printers (which the Acrobat PDF Virtual Printer is) that causes the issue you mention. There is a hotfix from Microsoft to fix this problem:
However, if you are seeing this under Windows 7 then something else is going on. I'd suggest you repost in the Acrobat forum if the hotfix is not appropriate for you. When you repost please add details (OS?, version of Acrobat? version of Powerpoint? If Helvetica is TT or PS and vendor.