Thank you all for your help and suggestions. It is working properly now but I don't really know why.
To test out the video card, I put it into another machine that was running a P4 and only 1Gb of memory. Even in that old machine it buffered even the 1080 videos instantly and had no trouble playing beautifully.
Then to test out the new build, I put a 9800GTX+ I had lying around and it still would not play HD videos. So figured I might as well put the right card back in and begin to test the drive, memory, etc.
But when I put the right card back in it now plays even 1080 videos perfect.
I am assuming that there was a problem either with the drivers, which needed to install each time I swapped the video card, or the card itself was not seated properly.
In the meantime, however, the machine did fail the "Network Throughput" test randomly while testing several times in QA. If this had anything to do with it or is going to become a problem in the future I don't know. I would still like to do some testing to find out exactly what went wrong, but I have already spent way too much time on this as it is.
Thanks again everyone and I have handed out some rep to all that helped.