Well dennis, there's slideshows and then there's slideshows. If you have a folder on your HD, outside of the iPhoto folder, there's a way to create a slideshow by opening up the folder and selecting all and right-clicking, then choosing Slideshow from the contextual menu. This will start a full-blown slideshow and it's fantastic!
If you mean can you create an iPhoto Slideshow from a folder of photos outside the iPhoto Folder, then you would either use a 3rd party app or just drag that folder from its location, straight to the Source pane in an open iPhoto Window. Then just click on Slideshow and start creating.
You can also use iDVD to create a slideshow of the folder if you want, then burn it to disc. This is AFAIK.