Without knowing more about your system and software it's going to be kind of hard to even know what application/s you are talking about.. Although, considering the information that you have provided; If the Rainmeter that you are talking about is the computer system monitoring software; it supports "transparency effects" so it may have to do with video drivers and media player overlay settings. Open up your video player and select options, video playback; then look for a settings that looks something like "allow video overlay" and try changing it and testing to see if that does anything.