An internet urban legend that's become infamous after the Sasebo Slashing in 2004. The killer (a schoolgirl) had bookmarked this particular video.
Here's the Wikipedia summary of the video:
Two friends discuss a rumor about an Internet pop-up ad that asks "Do you like -?" (あなたは-好きですか？ Anata wa-suki desu ka??). If someone closes the pop-up, he dies. The more skeptical friend does some investigation. At home, he researches it online but is unable to find anything about the site online. After he ends his search and begins looking at his usual websites, the message pops up, with a childlike voice asking: "Do you like -?
"
Even after he closes the pop up, it keeps re-appearing, repeatedly asking 'Do you like-?'. It then starts automatically reloading and the girl's voice becomes mangled till the boy eventually notices the question changed to 'Do you like red?'. The boy keeps clicking off the pop-up until a new part of the message is revealed, finally asking the completed question in a normal childs voice" "Do you like the red room?" (あなたは赤い部屋が好きですか？ Anata wa akai heya ga suki desu ka??). Then a website with a list of name appears, with the boy's friend at the bottom. The boy becomes aware of something behind him.
The next day, there is rumour and gossip at a school about two students who committed suicide, painting their rooms red with their own blood. Then, the protagonist's computer is displayed, with the protagonist's name at the bottom of the list. It then shuts off, along with an eerie sound.
After the video ends, if the user's browser has not disabled pop-ups, a pop-up resembling the ad from the story will appear.