I don't know whether this is just me but sometimes Minecraft randomly crashes and when I go back on the website it takes me to a random website. First time it was some Java page now I'm getting some Japanese Manga pop up. Notice the address box which still suggests I'm on the Minecraft website.
sounds like someone has been looking at too much porn.... get a free anti-virus program like AVG or something of that nature and do a complete system scan... just make sure its a reputable program, there is alot of fake ones that just make things worse...