Never buy it again.
You've bought it (though what you've actually purchased is debatable -- you certainly don't own any rights to the film, so really you're just renting it forever). So you should be able to do what you want with it.
All this 'it's costing the industry money' business is a load of rubbish. There still seems to be quite a few big budget films being made, quite a few rich actors, and a lot of rich company execs. Hardly 3rd world.
You need a DVD decrypter. DVD shrink apparently works well on Windows. To convert to a video format (mp4), try Handbrake.
The issue is that CDs, LPs, etc. aren't protected, but DVDs are (with several technologies). Googling for DVD decrypters will help though.