You can definitely run the computer display to the television and run your slideshow directly in PowerPoint. What you can't do (as far as I'm aware) is update the slide show in real time as its being shown. You'll have to exit slide mode and update it, then begin the presentation again.
If you run the monitor as a secondary display, you could play a commercial video in one screen while you update the slide show in the other. That way no one outside has to see you actually update it.
If I remember correctly, power point shuts off all monitors to display the slideshow. I don't think you can run it in show mode and still do your regular work from the same box. If anyone knows a way, I'd love to hear how.
Do you have access to a fairly cheap PC that you can dedicate to this purpose. All you need to do is install the power point viewer (free) and run it on a seperate box. It wouldn't be horrible to manage, especially if they're networked and you use VNC or remote desktop (heck, even a KVM would do the trick).