"Paranormal" is far too broad of a definition to disbelieve all-encompassingly. I believe there are things in this world/universe that we have yet to discover and/or cannot explain with science as we know it. I also, however, believe that 'most' of the "Paranormal activity" out there is human deception.
Aliens, I find it quite egotistical for us to think we're the only 'intelligent' species in the universe, and that Earth is the only planet which houses life. Have aliens come to Earth? Maybe, I rather doubt it myself. I believe most of the "UFO sightings" have very plausible explanations. Encounters? Well, it's possible some are real, but again I find it doubtful at best.