Sharing a Computer with friends is problem both for security of information and safety and welfare of your computer.
I have owned about 10 computers laptops in my Travel Life, and two of the major problems occurred because I loaned the computer to friends.
One time in Koh Phan Ngan, Thailand I loaned the computer to a friend, he was drinking beer and spilled a drop into the keyboard, successfully killing the motherboard.
Generally, if you loan a computer to a friend, it will come back to you with photos, programs and unwanted information.
The number one reason they wish to borrow or use is to arrange their photos, I never allow this because they put specific software needed for their camera. Then they wish to burn DVD or other mass memory devices.
The rule of thumb is this, if a friend wants to use your computer, you perform the task for the person, do not allow them to work on your computer, this is the only safe strategy.