This is a feature of Windows.
Right click on your desktop and select Personalize.
Click on Screen Saver on the bottom right of the window.
Uncheck "On resume, display logon screen"
Click Start and select Control Panel.
Click Hardware and Sound.
Click Power Options.
On the top left of the window select "Require a password on wakeup"
Select "Change settings that are currently unavailable"
Select "Don't require a password"
Both of these settings were set correctly for me, but I'm still having the same issue. (Where it locks on its own after a set amount of inactivity)
Edit: I should add that it just started doing this the other day when I changed my user profile to be the "owner" of my music folder so iTunes would stop giving me jerk-ish errors.