The problem they have is that high-energy particles can hit random bits in the memory, and possibly change them. As such they have a large amount of radiation s

The problem they have is that high-energy particles can hit random bits in the memory, and possibly change them. As such they have a large amount of radiation shielding, but some things can get through given enough time. They have a second computer that can "take command" in the case of any failure. The second computer is the same so the same programs can be used (obviously). The thing about Curiosity is that it left Earth with a very minimal program, and bits were sent to it on the journey. When it landed, it used a different program then it does now. Remote programming allows it to carry out a large amount tasks, with a minimal amount of hardware. The last thing is that everything is double, and triple checked. If something goes wrong, then it goes very wrong, so they ensure that everything is working fine. While the B computer is being used, they'll probably do a full bit-by-bit wipe of the A computer, then load software back on it.