Recent changes to iTunes has resulted in the bar being raised a little on the slowest machines that ran without issue. You should check your IDE DMA settings just to be sure.
I've purchased two episodes of LOST from iTunes, but just can't watch them because the video jerks along so much. I have an older computer (933 MHz P III, 512MB RAM, ATI Radeon 7200 32MB), so I'm not expecting perfection, but I used to be able to watch videos with no problems, and I'm able to watch shows on DVD through my DVD drive. I tried shutting down the obvious resource-hungry apps., slowing down my Hardware Acceleration in Display, but no help. Yeah I know, get a new computer! But outside of that, any ideas appreciated.