Create a shortcut on the desktop...
Right click anywhere on your desktop and choose "New..." then "Shortcut".
Enter the following text in the box
%windir%\system32\rundll32.exe user32.dll,LockWorkStation
Click next and name the shortcut "Lock Workstation" or similar.
The default icon is a bit naff, so right click on the new shortcut and choose Properties. Then choose "Change Icon". You can use any icon you like, however one that is suitable is a picture of two keys. Use browse to find "C:\Windows\System32\migpwd.exe" or "C:\Winnt\system32\migpwd.exe".
Once you have done this... you can hide that shortcut anywhere on the pc or on a server.... Go into that workstation, scheduled tasks, and add that icon to the scheduled tasks list... Pick any time, daily... When done, go into advanced properties... Set it to RUN WHEN IDLE for how ever many minutes you want. Ensure that is your only check... Test it... Should be good to go.
No screensaver, except windows default after so much time of inactivity after being locked.