1080P Blu Ray playback laggy

Hello,
I have an old HP DC7600 computer with a Pentium 4 ruing at 3.0ghz I believe (with hyperthreading) and a Quadro FX 3500 in it (equivalent to a 7950GTX I believe) that I am using as an HTPC. When trying to play 1080P Blu Ray video files, playback is stuttering. If I use the TOP command, it says the video player is using around 150%-180% CPU. Does this mean it's my CPU that's bottlenecking? I hope that it is just the GPU, as I have plenty of other GPUs to try, but no other available computers.

I would try using a proprietary video driver in order to check GPU usage, but installing Nvidia's drivers causes the entire system not to boot for some reason (possibly because the GPU is so old).