Why does my computer get choppy when watching certain movies?

Basically i use my comp to watch movies when i'm bored, and recently mainly on 2 720p and 1080p movies it has gotten choppy to the point where i see like 1 frame every 5 seconds..I did this on my old comp and it worked fine exept the same thing happened with avatar on 1080p, but every other one worked, now on my new one the 2 hi def movies i got don't work either, i thought it could be the fact they're hi def and maybe it ws to much on the video card, but the same thing happened on my last comp with just one movie, and the others work, They are all downloaded, no streaming, all on a blueray player i have, and this is the first time, so far no hd videos work on this pc, but i've only tried 2, on my last comp only 1 didn't work and others did. any ideas?