I'm hoping you don't mean trying to return it to the form of its uncompressed master. MPEG is lossy, so you'll never get back what was lost in the compression process. You could put it into VirtualDubMod, and save it as an AVI.
If you are referring to splitting it up into elemental streams, I like to use Vobrator, but there is a bit of a learning curve. If you had DVDLab, you could just drop it into the Assets box, and it'll automatically split up the whole VOB (which could be problematic if you just wanted one small section. You could crop the section in DVDShrink first.)
There are many different ways, but these are the tricks I use.