As explained in section 3.1, if you include a PostScript image in your PowerPoint presentation, it will be necessary to save the slide as PostScript. However, this complication may be avoided with a simple technique. Simply rasterize the PostScript image at a high resolution, and include the resulting image in your slide.
There is a program called pstopict on the SGI which does exactly this. The program takes a PostScript file which is assumed to contain a single image. The PostScript is rendered at 300 dpi, and the result is written to a Macintosh PICT file. This PICT file may be included directly in your PowerPoint presentation.
It is not generally worthwhile using this technique for slides created with Showcase, since the slides must be exported as PostScript anyway. However, there is a program similar to pstopict called pstorgb that creates an RGB file.