It is likely that your source files are of a type that WMM does not like.
For example, my sources are .vob files which I convert to .wmv files for
import into WMM. You can get better advice at this web site:
http://www.windowsmoviemakers.net/forums/
"LKF1988" <> wrote in message
news:696623F7-CDCB-45DE-BB71-...
> So I've been working with Movie Maker since the beginning of XP, and I've
> never had this problem until Vista so something's up.
> My project will periodically refuse to play, i.e. the timeline won't play.
> This occurs when either I add another clip to the timeline, when I add a
> transition, or when I quit the program and enter in later. I hit the play
> button and nothing happens. It is not frozen because I can still play
> other
> clips, just not my timeline or anything on my timeline: music, effects,
> video
> clips, titles, etc. I have to delete half my project to get the timeline
> to
> play, which gets really annoying when you're working on something for four
> hours and you have to erase and re-do half your work.
> Please help.