The problem with this is, that if windows pops up an autostart dialog (if you plug in a USB Stick or mobile phone/ Tablet for charging, CD/DVD) or if Avira pops up his stupid commercial window, MP loses the focus and the autostart/windows dialog is in front of MP and you need a mouse to get MP back in front.
I am not aware of any Windows settings that allows the screen saver to kick in when some fullscreen form has always on top set.Is there any possibility that the windows screensaver can be used if the "always on top" option has been set?
Avira? Hey, they want to make business that's why they nag you on purpose. They have to make a living, too. Use a different product, buy a license or even pirate it. There are enough options here for anyone.