Is this about the possibility of someone malicious altering your code to do something you didn't program it to do? Or is it about the possibility of random errors messing up your computation? In the latter case, you probably don't have to worry because the error rates on modern computers are something like 1 in 10^17 - that's less than one per processor per year, and if even that is intolerable you can use error-correcting algorithms to reduce the effective error rate as close to zero as you want (at the cost of needing more time to do a given computation).
If it's hackers you're worried about, though... there's really no expectation of security at all. Someone in physical control of a computer can, in principle, modify it to do absolutely anything that can conceivably be done by a computer. They could disassemble your code and alter assembly instructions at will, if they want, to make your program behave however they want it to. We don't usually worry about this in practice, though, because most of us aren't writing anything worth the effort to hack. Those who are, e.g. programmers designing military encryption hardware or nuclear missile control chips, can fall back on tamper-resistant hardware which makes it extremely difficult to alter the code. (That stuff can be expensive, though)
You might want to take a look at a book called "Security Engineering" by Ross Anderson, which describes some of this tamper-resistant hardware, and in general how people who really need to secure their code can do so.