Laptop doing weird things (files missing)

I have a windows 7 laptop. It was at home on the weekend, connected to the internet, no password  . Nobody was home. When I came home, and went on, I noticed that a bunch of files were missing, and I have not been able to find them them. I have Norton Internet Security installed, and I ran a scan with malwarebytes which also did not find anything. The funny thing is that I have noticed other random little strange things going on. is it possible that someone is logging in remotely without me knowing? I'm getting really sick of this happening. I have event viewer logs from the weekend, and I can run hijackthis etc, I just want to get over with this..