Quickest? Don't know for certain. There are many different programs that can use divx, including Dr Divx, AutoGK and Fair Use Wizard. There are many others that can convert to Xvid (which is very similar and maybe slightly superior to Divx). AviDemux is one of those, and Xvid4PSP.
They all have one thing in common. You need to start with unencrypted dvds, which means you should decrypt and rip commercial dvds to the hard drive first.