Here is a quick tip:
When a friend sends you a pps file which by default starts slide-showing the presentation in PowerPoint (Ehud Kenan complained about such attachments in his Hebrew blog, and I commented on how this can be easily avoided).
Often, these presentations has an accompanied background music, which can be rather annoying, especially if you have other people sitting near you.
I often thought Microsoft should have added a mute button to slide shows of such presentations, but until they do, I found out that the easiest solution to silent these slide shows (other then temporarily muting the computer or turning the speakers off) is simply to Alt-Tab to another application and then back to PowerPoint. These makes PowerPoint stop playing the music, until the presentation’s slide show is started again.