To answer this question, the 'term' alien needs to be properly defined.
If by alien it is supposed to imply 'extraterrestrial life forms' whatever they may be, including atoms, and unknown materials, then the probability of having any in one of the millions of solar systems out there would be very very high. The conditions that made Earth a medium that life can exist on, can very well be met in many of other solar systems that we know very little about. And besides, all our scientific discoveries have been made through Earth's medium of life concept, but in other solar systems, conditions for life to exist may be different, and this is where I believe the real scientific challenge lies.
However, if by alien you mean something that looks like this guy:

... then probably not. But I am a firm believer that lifeforms alien to earth have developed themselves into something we cant understand, or rather goes beyond our scope of thought, therefore the notion of these lifeforms reaching us before we get to them is highly likely.
The debate can go on for ages, and is very interesting, but we are ages away from getting any sort of answer for this question since all man made technologies at the moment can only take us so far, and that isnt close enough to an answer!