I can certainly understand why you find the situation troubling. Feeling safe in your home is very important--it's your base of operations and, ideally, a place of comfort. The situation, however, may be what you fear or it may not be. Let's examine this more closely.

Try to think back to the time before you found the download. Did you feel uncomfortable around your stepfather then? Have you found other things that would indicate an interest in child pornography or in stepdaughters in particular? Has he touched you or stared at you inappropriately at all?

Though his actions may be troubling to you, without other indications, his downloading this video clip may not have the meaning you think it has. Your stepfather may have downloaded the video by mistake and neglected to delete it. Or he may be preoccupied with youth in general and so may have been attracted to the taboo-nature of the topic even if he had not had a stepdaughter.

Regardless, leaving the file on the computer was definitely a lapse in judgement on his part. His actions have now planted a kernel of suspicion and fear that may be difficult for you to squash. If you feel that this download truly does not have to do with you, ask yourself whether you can let it go. This may take some effort on your part and an understanding that some people use pornography as they would fast-food--with not much thought about its content or how it might affect them (and, in this case, others) down the line.

If you do have trouble letting it go--if you become so preoccupied with the situation that it affects your mental health--I would recommend biting the bullet and asking your mother to address the issue with him directly.

I noticed that this was not an option you seemed to consider right away. I wonder why. Do you and your mother have a distant or conflicted relationship? Have you been trying to protect her from the situation or avoid family conflict? These are all understandable concerns.

However, the truth is, the conflict has already been introduced. If you are persistently uncomfortable in your home as a result of this "discovery," something must be done to resolve it. Addressing the topic by saying that you found something troubling, you don't know what it means, but you'd just like to clear the air, may help everyone keep the situation in perspective. If your mother is definitely not an option, I would recommend, as an alternative, speaking to a guidance counselor or another older relative or family friend. Talking about this with an adult you trust may go a ways toward easing your mind.