Powerpoint is a horrible horrible program. I advise trying to avoid using it at all costs. Consider writing presentations in XML with a nice Extensible Stylesheet to render. That way you can properly separate the content from the presentation.
Visited links in Powerpoint appear by default as a nasty grey/blue colour that isn’t the most readable on a digital projector. To change this to blue:
Format ->Slide Color Sheme -> Change the colours of Accent and Hyperlink and Accent and Followed Hyperlink