If you don't need full control over every transition and effect in your slideshow, Animoto is the way to go! This online program will create awesome slideshows for you. All you have to do is upload your photos and music, or choose some of theirs, and that's it! Animoto does all the hard work for you! It will set the picture timing to the music's tempo, insert transitions and do all those things that would take you hours to do in Windows Moviemaker. You can also include movie clips and adjust picture pacing to half speed or double time if you'd like.
You can create 30 second videos for free or buy membership that allows you to make longer videos, and even unbranded ones. You can also just purchase one full length video if you'd like.
As a teacher, you can sign up for a free Animoto for Education account that lets you and your students create longer videos at no cost. The branding is still there, but it's not terribly intrusive.
I'm in love with this website! Check out the little video I put together: