Save this as activate_screensaver.vbs and when you want to instantly activate your screensaver and also having it require username + password to deactivate just double-click it.

Set objShell = CreateObject( "Wscript.Shell" )
' The "True" argument will make the script wait for the screensaver to exit

returnVal = objShell.Run( "%windir%\System32\ssmypics.scr" , 1, True)
' Then call the lock functionality
objShell.Run "%windir%\System32\rundll32.exe user32.dll,LockWorkStation"