Sony vegas issue with preview the video HELP!?

Iv been trying to figure this out all day. I wanna make a montage and im using sony vegas to edit my clips. On the preview on sony vegas it says its all in sync and it goes how i want it too. When i click 'Tools' 'Preview in player' I wait for it all to render and then watch it in the windows player but it all goes out of sync for some reason and the clips all muddle up with each other?? HELP :( ?