In reply to MikeR:
As no one else has asked.... what spec is your machine? (processor, Memory (RAM), HDD size and percentage disc space free?)
what sort of videos are you playing? HD? 4K?
Press the windows key + R together - you should get the "Run" command line prompt pop up.
type "shell:games" press enter and the games shell should pop up (it might ask about default folder display settings just click default option) in the bottom right of what will appear to be a blank window (unless you have lots of app games) you should see a number (mine is 5.9) next to "current system" - this is the Windows experience index and gives an indication of overall performance - what is yours showing? - This will give an indication if your hardware is an issue