Does this happen only to video files or something else too?
Or does it happen to a certain video format?
Try a different player, try updating the player
Try updating your video card driver
Try an update to the codec pack.
Try a scan with Malwarebytes just in case, and a cleanup with CCleaner. (just google them, download and install)
See what backgraund services are running that consume too much. Disable anithing not vital from Start>Run>msconfig (services and startup)
If it happens to more stuff, try a system restore