Ok, I have several thoughts about this, but it seems pretty obvious: Do something about it before whatever happens.
1. If it's a prank, then your going to be dead! For now, simply move it to another place where no one can find it. Maybe hide it somewhere on the C drive where it will be hard to find (eg. In a games folder).
2. Change your laptop password. This will stop your brother getting on to it. So if it shows up again, and you know your brother hasn't been on it, you know it's a virus. Oh and hide your laptop, or always keep it with you - so you defiantly know he's not on it.
3. Do a virus scan on your computer and stuff. Just to make sure... You may also want to look in your internet history while its scanning, to determine where it is coming from (if it is your brother).
4. In the end... Tell your parents about it. It may be a bit embarrassing, but it's not actually yours is it! Tell them that your finding homosexual pornography on your laptop, and it's not yours. You may also want to explain your brother used to know your password, so your worried that it was him.