Please please! computer people! Why was there a random person's email address on my computer?

So I left my house last night in a hurry and never logged off my computer and when I got home this morning, My computer needed to be restarted because there is a sysinternals fake virus software popup constantly coming up. so I closed out all windows and resarted my computer, when I tried to get internet service, I was connected but My home page wasn't there. It said Blank page. then I typed in bing and my homepage came up, but there was another person's email typed in to my hotmail. it said holdenmanuel@hotmail.com. My email is usually in tht box. What is happening? I do not understand. I have been staying with my grandparents temporarily and no one was over and my computer is in a room by itself. my grandparents would never touch my coputer I am positive about that and anyway we do not know a manuel holden. What is going on? I am scared my identity is being stolen