You can lock the PC with Windows-key + L when you go away. Be sure that all Windows accounts on your PC have a password.
One of my friends frequently comes to my house for copying games ,files , software, movies etc. and the problem is if someone (like mom or dad or someone else ) calls me i have to go or he gives me some work of like scanning or showing me any book or somethings like that . the problem is i do not want to let him copy some files or documents without my permission so i think if there is any software using after setting password , if someone tries to copy any file he will require password to do so .or if there is any other way you know will be good please do tell.