High Quality video plays badly?

I got a Samsung N150P laptop with 1Gb ram and Windows-XP (SP3). Recently all “High Quality” videos (normally around 1.3Gb of file size) are unbearable. On VLC (2.0.5) they are choppy, semi pixlated and shows/freeze in steps of ¼ of seconds. On Potplayer (1.5.32) the sound and video synchronization flow during the movie to up to 3–4 seconds difference. I changed the PC advance performance settings to be geared towards Performance (and not Looks) but with no change. I tried to restart the PC and only play the video (as only application I run) with no better result.
Could anybody please advice of the reason for this?
Thank you very much.