Hi!
Whoops! Talk about an error of judgement. We all make mistakes. The best policy is always honesty, but Mary Poppins always said a spoonful of sugar helps the medicine go down. I'd tell the interviewer you were dismissed for misuse of company property, and if they ask you to elaborate, simply say you made an error in judgement and used the company PC to view material that wasn't directly related to your job.
You don't need to say more than that.
Good luck in whatever you do next!