This problem usually occurs when Windows has not properly closed the default application that it uses to run videos. This is easily fixed. One option is to reboot your machine (complete shut down and restart – not hibernating or sleeping). Another option is to use CTRL+ALT+DELETE command to invoke the 'task manager' You may or may not see the video program running but if you check the 'processes' tab you may see that the process is still running and choose 'end process' to completely shut down the video application.
NVivo cannot access a file that is being used by another application (no program can) so this is why you are getting this error. Let us know if this solution works for you and if you have any more problems – we will investigate further.