You can change your Windows 7 settings such that the PC is locked automatically after the PC being idle for some time.
To change this, follow the below steps
1. Right click on your Desktop and click the Personalization.
2. In the Personalization Setting Dialog, click Screen Saver.
3. In the Screen Saver Settings, check the option “on resume, display, logon screen” and provide the time to wait and click ok.
Now after idle time selected, you should see the login screen.