Access Controller is a software designed to avoid this type of intrusions. It blocks your Windows session so nobody who doesn't have the password can use the computer.
First of all, you have to set up a secure password and authorize the users who'll have access to it.
After you configure that, just activate one of its two block modes: one that keeps the Windows desktop visible (although not accessible), and another that just runs the screensaver you have set up.
Finally, Access Controller also has an interesting utility that lets anyone trying to access the computer when it's blocked leave a message for the system administrators.
You have 30 days to try the program.