This machine is not very speedy but it is more than capable to handle dvd playback I am quite sure
Correct. Windows handles the DMA and optimizes your drive already, while Linux merely plays it safe and only turns on one or two things.

Once we use hdparm to turn on a few safe settings (let me know if you need help with that) on both your dvd rom and hda, not only will your dvd not skip, but a general speed in OS preformance as well. Remember to add the entire hdparm + parameters to your rc.local so they set themselves each time.

note: your cd writer since it is scsi emulated can NOT use hdparm