There are two common scenarios where PowerPoint fonts cause a problem in webinars:
1) Creating the PowerPoint on one computer and displaying it on another
2) Uploading the PowerPoint to a web conferencing product that converts and stores the slides in the conference room for display to participants
In both cases, you have the potential to see discrepancies in the text on slides. One of my clients creates their PowerPoints on a Mac. When I open their files on my Windows computer, the bullet symbols that they see as hyphens show up as little mailbox icons. I have to manually change the bullet symbol selection to get it back to a hyphen on Windows.
If a PowerPoint creator uses proprietary or non-system fonts, there is no telling what may happen when they ship the file to someone else. Even though PowerPoint gives creators a way to embed the fonts in the presentation, it still may not work. If PowerPoint can't find a font on your computer, it just changes the text to a different font for you, usually causing problems with incorrect line wrapping in the process. If the presentation doesn't look right on your screen, it won't look right when you use screen sharing to display it in your webinar!
Situation number 2 in my list is a more special-case scenario. Certain web conferencing products use an alternative to screen sharing for showing PowerPoint slides. You upload your PowerPoint file into the conference room and the software goes through a conversion step (usually turning it into a Flash or HTML5 "movie" internally - although you don't know that).
The PowerPoint may look fine on your computer… You have all the fonts installed, and everything shows up as expected. But when you try to display your slides in the conference, you see text converted to other fonts. That is because some of the most popular conversion algorithms have no way to pick up embedded proprietary fonts.
Fig 1: Fonts as they look in PowerPoint on my computer
Fig 2: The same slide uploaded and displayed in a web conferencing product
With that background behind us, we are ready to look at a "quick 'n dirty" brute force workaround when this happens to you and you can't think of any other way to solve your problems. This solution has a major caveat that you need to be aware of, but a lot of times it will save you in a crisis.
The short description is to save the presentation as a set of pictures, one for each slide. Then you create a new presentation composed of the slide pictures.
STEP BY STEP INSTRUCTIONS
1) Open the PowerPoint on a computer that displays everything properly. Choose "File - Save As" and select the option for JPEG File Interchange Format (*.jpg). Check my notes farther down for an explanation of why this is your best choice.
2) When asked "Which slides do you want to export?" choose the button for "All Slides"
3) Designate a destination folder for the save. PowerPoint will create a series of files named slide1.jpg, slide2.jpg and so on.
4) Open a new PowerPoint file and make sure the slide aspect ratio is the same as your original (4:3 or 16:9). Add a bunch of blank slides… The same number as you had in your original.
5) On slide #1, choose the command to "Insert Pictures." Choose slide1.jpg from your save folder. Page down and repeat the process for each slide.
6) Save the new presentation. You are done. This version can be uploaded or shared through screen sharing on any other computer, with no chance of text getting replaced (since you have eliminated all text elements).
UPDATED May 29: Huge thanks to Chantal Bosse for suggesting a faster insert method for steps 4 and 5. Just use the "Insert Photo Album" command to insert all the pictures at once. You can avoid having to create blank slides and insert each slide picture separately.
IMPORTANT CAVEATS AND CONSIDERATIONS
This is a "frozen in time" version of your PowerPoint. You cannot make any last-minute edits and alterations to individual elements, since each slide has been flattened to a single picture.
You lose all slide transitions and animation effects from your original presentation. Each slide is just a static image. You can always add slide transitions back in the new file if desired.
Some "upload and convert" webinar tools reduce the resolution or image quality on pictures. Your mileage will vary. Make sure to test how it looks in your webinar program.
OTHER NOTES AND SUPPORTING DETAILS
Your current "view size" does not matter when you save your slides as pictures. Whether you are viewing at 40%, 100%, or 200%, the pictures are saved at the perfect size to fill a slide - 960x720 pixels for older 4:3 aspect ratio presentations, and 1280x720 for widescreen 16:9 presentations.
JPG gives you the best compromise of image quality vs file size. I tried tests with GIF, JPG, and PNG. GIF has noticeably lower image quality and a slightly larger file size than JPG. PNG may result in microscopically better image quality, but I am not convinced that it is not just expectation bias. I find myself squinting and asking myself if there is really a difference. And PNG files are close to three times larger than JPG. Your final presentation size will be noticeably different.
Inserting pictures into your new presentation will go a LOT faster if you add the Insert Pictures command icon to your Quick Access Toolbar. It's faster than using the keyboard shortcut of Alt+N, P. You can get very fast at "click icon, choose file, press page down."