How can I stop from downloading porn?

So my brother who is 19 I'm 17 keeps downloading porn on my computer. His computer is being repaired so he is using mine. Now my computer is running so slow since he keeps downloading so much porn. It used to be fine before and fast now it's running so slow. What should I do? Should I just confront him? 

There are many ways to block the porn websites..etc and technology levels increased the security system has been grown same beside it.

You can always put a parental lock on the computer, basically it lets you restrict which websites and programs the users on the computer can use, mean you can set only few websites that he can access. You need to make sure you're a administrator of your computer and you have a user account for your brother, which means limited account or guest account. This feature only works if your brother logs in using the user account, so don't leave your administrator account on or tell him the password.
In this way, you will get a every weekly report on the websites and programs he has been using......

Follow this way to enable it:

Firstly Logon to your Administrator account

1.Click on the bottom left-hand corner on start button.
2. Open 'Control Panel' from the sidebar
3.On Windows 7, you should see 'Users and Family safety' on the top right, If you're using a different windows version, search "Family safety" on the search bar and it should come up.
4. Once you're on the Family and safety, it should be simple from there onward.

try any other antivirus program, that also helps to prevent accessing the porn websites.