Why does Cobb use the spinning top?

I do not believe there is any mention of what his original totem was.

I think we are meant to presume that he adopted Mal's spinning top totem when she committed suicide in what Cobb and we are meant to think is 'the real world' in the story. We are told that Mal invented the totem concept. It could be the case that at the time Cobb and Mal are in Limbo that only she has a totem and that he has not yet adopted that idea. However I think it is fairly safe to assume that as soon as she invents the concept of the totem, that she shares the idea with him and that he has his own totem in Limbo and at the time of her death.

Therefore we can safely assume that he believed he was awake at the time and was using his own totem to validate that state. He seems strongly to believe that they are awake at the time she takes her own life, which appears to support the idea that he has a totem of his own at this point. His thinking might be that since he knows he is awake, he would then be able to adopt and use Mal's totem safely from then on. This is the only answer I have to your question - he took and used the totem when he could validate that he was already in the real-world, and trusted that he could then use the new totem.

However this is one of many points in the movie that are (I believe) intended to make us wonder whether he is still in a dream in the 'real world' at the start and end of the movie. The argument that he could use his own totem to validate his awake state before taking and using Mal's totem seems fairly watertight, particularly as it has an easy to interpret sign (if it does not stop spinning he must be in a dream).

However conversely this easy to interpret signal is also a weak point in using the totem that she designed. Everyone else's totem has something about it that only they understand - Arthur's loaded dice, Ariadne's weighted chess piece. One can imagine that anyone could dream the spinning top falling over, perhaps Mal had a more detailed and subtle understanding of the top that would enable her to distinguish it falling over in someone else's dream.

I am therefore guessing that at the time he took the top, he was in the real-world as he was using his own totem at the time. However Mal's totem because it was not of his design is an unreliable guide, and that we cannot assume that he was in the real-world at the start and end of the movie.