Which processor does your computer have on what operating system? Which version of the program are you using?
What model of camcorder is your video coming from and what format and resolution is it?
Have you ensured that any photos you've added to your project are no larger than 2500x1850 pixels in size (at 72 ppi)?
Have you ensured that any music or sound files you add are WAV files?
Have you rendered your timeline (by pressing the Enter key) before playing it?