Are you talking about viewing it in the preview windows in Movie Maker?
If so this is a low quality rendering of your slideshow and is not what the
finished project will look like.
If you are talking about a finished video being viewed in Media Player then
take a look at the settings you chose when saving the video. DV-AVI is best
when working with video but if the project is all still images DV-AVI tends
to be pixilated, blurry, stretched etc... so in those cases one of the WMV
settings (I always use High Quality NTSC) is better.
Also if you are only working with still images and music you may want to
check out Photo Story 3 as it was more designed for this function and has
far more options and control over transitions.
PS3:
http://www.microsoft.com/windowsxp/using/digitalphotography/photostory/default.mspx
-Wojo