I currently use a PCLOS 2012 KDE clean installation on a (chinese-rebranded) laptop with a NVidia GeForce 9300M GS GPU.
I have a persistent problem. When visual effects are turned off,starting a video with VLC, Mplayer or using Skype video freeze immediately everything, while the sound continues.
Furthermore, when visual effects are on, although video plays, trying to fullscreen any of these apps (I suppose and others as well) gives the same result. Any ideas?
First let me say that I'm not familiar with your particular video card, and that Nvidia's equipment is generally good for Linux, but some series can be a bit temperamental.
Good thing that you tried different video players. Shows the problem is not specific to any one of these, but resides in a layer that is common to all (or, at least, the ones you've tried so far).
I'm wondering if it does this with all video sources. Try different video sources (i.e., a different video file). I know that my laptop will do exactly what you're describing when trying to render anything at 1080p, fullscreen or no. I ran into this while testing files encoded at Hi10P (10-bit depth x264).
You might try disabling Desktop Effects and see if that has any (excuse me) effect on the problem. Wait... I see you've done that if I'm reading correctly... hmm.
Have you tried another desktop environment (e.g., LXDE, XFCE)? If not, give it a whirl. Boot from a LiveCD or just install it alongside your KDE install from Synaptic and change session type when you next log in.