Individual blocks can be hammered out and replaced - so a whole new wall does not need to be poured (unless the ENTIRE wall is crumbling.) If it's just a few blocks, or a limited area, they can be removed and replaced.
Also, depending upon how badly they've deteriorated, they may be OK as they are with some tuckpointing & maybe some I-beams.
This is probably not a DIY job - call a mason.
Also, what Jay said about landscaping and drainage is very true and very important.