How to invert your slides for printing:

Download and install the following generic printer driver:
Windows 95/98: http://www.adobe.com/products/printerdrivers/windows.html
Windows NT/XP: http://www.adobe.com/support/downloads/product.jsp?product=pdrv&platform=win

Instructions can be found here:
http://www.adobe.com/products/printerdrivers/pdfs/installanduseps.pdf

After installing the driver a new generic post script printer is available.
Under: 
	settings -> advanced -> post script options 
you will be able to select 
	negative output