Microsoft's "Thin Client" concepts (Eiger and Mönch) were last heard of in 2005 and seem to have sunk without trace.
Linux of course has a whole variety of free thin-client offerings that could handle Powerpoint presentations.
What with people pinching your gear and clients pinching your presentations you seem to have a lot of security worries... Do you frisk them for cellphone cameras?
Powerpoint Viewer will run on 98SE/XP and anything from a P3 on up. A few junkers behind a stout partition will do it for you.
Alternatively, your local welding shop could build you half-a-dozen cheap bolt-to-the-floor cages for the PCs.
Mac