How does the creative process of Biological Evolution work?
Your basic problem is indeed a fundamental misunderstanding of how DNA and by extension its evolution works. First of all, there is no such thing as "correct" or "error", there are only changes, some of them are better for the individual and some are worse. You also have a basic misunderstanding of the concept of information. An error does not decrease the amount of information, it just alters that information. For example, consider this sentence:


  The book is read


Say I make a simple error when I transcribe it:


  The book is red


The two statements contain the same amount of information. Nothing has been lost, only changed. But that change created new information. Now what if I try a different change:


  The book is rled


Now, this one is nonsensical, indeed, but how does that translate into DNA?

The computer analogy is a bad one. Programming languages have a very specific set of rules and anything outside those rules results in an error. For example, while this is a perfectly valid statement in many scripting languages:

print "Hello World"


This one is not:

priint "Hello World"


That, however, is not the way that DNA works. DNA is read in words of three characters, the codons. Every single possible 
combination of those characters actually has a meaning. Therefore, a mutation like that above which would decrease the information content of the sentence (here, the gene) is not possible. Let's quantify. The DNA sequence consists of a long series of chemicals, commonly known as A, C, T and G. Since these are read in "words" of three "letters", this means you have:

$$4^{3} = 64$$

64 possible combinations. Of these 64, 61 are words with a specific meaning and 3, the STOP codons, are punctuation marks, full stops if you will. In other words, all possible combinations of letters have meaning in DNA.

Because all possible 3-letter combinations have meaning, no change will affect the information content of the "sentence". All it can do is change it. Now, changing the information can indeed result in "sentences" that make no sense. In the DNA world, this most often results in what are known as deleterious mutations. These happen all the time, but the result is usually a dead individual. Those with such mutations are not even born (speaking of mammals).

However, other mutations, such as the change from read to red that I mentioned before, can give rise to a change of meaning, this change, if it turns out that it makes the individual carrying it more likely to reproduce, will spread across the population and will be selected for. That is how new traits can arise.

In closing, another point that you have misunderstood is that evolution is not a path from the least to the most complex. In many ways it is the inverse. In fact, by any measure that counts, bacteria are far more "evolved" than humans for the very simple reason that the only way you can measure evolution is in terms of numbers of generations. Think of evolution as happening every time a species reproduces, this means that bacteria have had orders of magnitude more time to evolve than mammals have. Still, this is a useless comparison which is why nobody compares species by how "evolved" they are. For a discussion on that, have a look here.