What is the correct pronunciation and spelling of "asterisk"?

It is not just you, and it is a fairly common error in pronunciation in many parts of the English-speaking world.

Here is an interesting usage note on asterisk from the New Oxford American Dictionary (2nd Edition on my computer):

Avoid pronouncing this word |ˈastəˌriks| or |ˈastəˌrik|, as many regard such pronunciations as uneducated.

and a related one from the Oxford Dictionaries Online:

Even though asterisk does not look like a tricky word to pronounce, it can be problematic. In both its singular and plural forms , it is often mispronounced as if it ends with -rix. Pronounced correctly, it ends with a -risk sound when singular and -risks when plural

The phenomenon is called metathesis, and it is actually not that uncommon in English and many other languages. Note how you say three, but not threeteen or thrid; you say thirteen and third instead. That's because thirteen and third have undergone metathesis from Old English þreotene and þridda.

Many languages have words that show this phenomenon, and some use it as a regular part of their grammar (e.g. the Fur language). The process of metathesis has altered the shape of many familiar words in the English language, as well.
[...]
Metathesis is responsible for the most common types of speech errors, such as children acquiring spaghetti as pasketti. The metathesized pronunciation of ask as ax /ˈæks/ goes back to Old English days, when ascian and axian/acsian were both in use.
[...]
The process has shaped many English words historically. Bird and horse came from Old English bryd and hros; wasp and hasp were also written wæps and hæps.

So, to answer your question, yes, that's a natural change that happens in many languages. Wikipedia even expressly mentions asterisk → asterix /ˈæstərɪks/ as an example of metathesis, along with many others. It can start off as a slip of the tongue, or as deliberate "laziness" if you will, but once the "wrong" pronunciation gets sufficiently established among native speakers, you can no longer call it wrong from the linguistics standpoint. As one of our linguists once put it elsewhere, "metathesis can be done in error, but ultimately, if the alternate form becomes established in some dialect, it is hard to argue that every person doing it is making an error." Again, not many people nowadays would argue that thirteen is a speech error. Whether or not the same will happen to asteriks, is, of course, pure speculation.