I'm guessing that the reason that the grout is crumbling is that it was mixed improperly. I would recommend removing the old grout and regrouting. To remove the old grout you could use any number of abrasive products available at your local home center. (just ask in the tile department) You might even try one of those rotozip type tools with the appropriate attachment and grinder. Either way, it is not an easy project and will take some time. Once the old grout is removed the grout spaces should be thoroughly washed (rinsed out) to remove all of the old grout dust (it inhibits the new grout from properly adhering to the tile). Applying the new grout. Mix up some floor grout with latex additive solution.(the tile department should have this) The floor grout is called "sanded" and stands up better on horizontal applications, like floors.
The wall tile should be grouted with non-sanded grout, also using latex additive solution. Good luck!