Slideshows can run into a variety of problems; try these solutions:
Some transitions, like Cube and Flip, may not work on older Macs with less capable video cards.
Music purchased from the iTunes Music Store and used in a slideshow can be heard only on authorized computers. Either pick different music or convert tracks (burn to CD and re-import into iTunes) for use on other computers.
If slideshows look wrong, try switching to "Thousands" of colors in the Displays preference pane.
If a slideshow takes a long time to start, it may be because of a very large music file you've set to play. Pick a smaller file to speed start time.
If slideshow transitions are slower than you've set, it may be because your photos are too large or your screen resolution is too high. Setting a lower resolution in the Displays preference pane or using smaller photos should speed transitions.
If you can't see your iTunes music library when selecting music for a slideshow, open each iLife application in this order: iTunes, iPhoto, iMovie, and then iDVD.
If you can't play music purchased from the iTunes Music Store, upgrade to the latest version of QuickTime. Get it from www.apple.com/quicktime/download/.
If a Windows user sees error -8992 when trying to play your QuickTime movie, have them turn off DirectDraw Acceleration in the Video Settings screen of the QuickTime Settings control panel.