What is the ruling on using my computer to download and watch movies and songs, knowing that I make it available for anyone who wants to use it?

If the device belongs to you, then prohibited materials must be prevented from being uploaded onto it, otherwise, you would be a partner in sin; due to Allah’s saying: "And cooperate in righteousness and piety, but do not cooperate in sin and aggression." Use a gentle approach by informing him that his action is forbidden, that you do not wish for either of you to fall into disobedience, and that you both cannot withstand Allah's wrath and punishment, and that you desire good for him by preventing him from this reprehensible act.