So... Why does not this work?

Code:
writevalue('HKCU\Control Panel\Desktop\','Wallpaper','c:\pic.jpg','REG_SZ')

$nul = UpdateUserParameters()


function UpdateUserParameters()

  SHELL 'RUNDLL32.EXE user32.dll,UpdatePerUserSystemParameters'

  exit @error

endfunction

$nul


It has perhaps something to do with the UDF-calling thingy, but I only got a bunch of errors. =P Remember I am a newbeginner in kiX'ing. =D