@echo off
setlocal EnableDelayedExpansion
del D:\ScreenSavers\Random_Screensaver\__RANDOM.scr
D:
cd D:\ScreenSavers
set n=0
for %%f in (*.*) do (
set /A n+=1
set "file[!n!]=%%f"
)
set /A "rand=(n*%random%)/32768+1"
copy "!file[%rand%]!" D:\ScreenSavers\Random_Screensaver\__RANDOM.scr