I've installed the application and repeated the procedure by adding a video file and everything worked normally. There were no lagging issues or something that might cause playback problems. By default, the software uses Windows Media Player to preview the files, therefore I recommend checking the video player to see if it's working just right.
Additionally, you could reinstall the application, through the following link:
http://vsdc-free-video-editor.software.informer.com/download/
Note: Make sure to download version 5.5.0.601.