I think you are trying to alter the wrong keys/values.

code:
Function SetAutoProxy ($Proxy)
   ; This function sets the corporate autoproxy for the client
   dim $Proxy, $key, $rc_write, $RC1, $RC2, $RC3, $RC4
   $key = "HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Internet Settings"
   $RC1=WriteValue($key,"ProxyEnable","0","REG_DWORD")
   $RC2=WriteValue($key,"AutoConfigURL",$Proxy,"REG_SZ")
   $RC3=WriteValue($key,"ProxyServer","","REG_SZ")
   $RC4=WriteValue($key,"ProxyOverride","","REG_SZ")
   if ($RC1+$RC2+$RC3+$RC4)>0
      $ErrorState=1
      WriteLog("Error setting AutoProxy registry settings. 0=success (RC1=$RC1, RC2=$RC2, RC3=$RC3, RC4=$RC4)")
   endif
   $file = "c:\Program Files\Plus!\Microsoft Internet\CUSTOM\install.ins"
   If Exist($file)=1
       $rc_write=WriteProfileString($file,"URL","AutoConfigURL",$Proxy)
       $rc_write=WriteProfileString($file,"URL","AutoConfigJSURL","")
       $rc_write=WriteProfileString($file,"Proxy","HTTP_Proxy_Server","")
       $rc_write=WriteProfileString($file,"Proxy","FTP_Proxy_Server","")
       $rc_write=WriteProfileString($file,"Proxy","Secure_Proxy_Server","")
       $rc_write=WriteProfileString($file,"Proxy","Gopher_Proxy_Server","")
       $rc_write=WriteProfileString($file,"Proxy","Socks_Proxy_Server","")
       $rc_write=WriteProfileString($file,"Proxy","Proxy_Enable","0")
   Endif
Endfunction
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