I ended running the scheduled task in Run only when user is logged on mode but w/ wperl as an interpreter instead of good old perl.

The code is launched, the wallpaper is set and the registry entries modified w/o a single command window poping up.

BTW, the registry editing is to activate tiling for the wallpaper, mandatory in case of a composed wallpaper on multiple monitors setup.

Thank you all for you help, especially the info about the Win32::GUI module.