The first code that I ever open sourced on gamedeception.net

injection_thread.cpp

// Some dll injection code
// November 21, 2004
// by SpuN [ http://spun.gamedeception.net ]
// injection_thread.cpp


DWORD WINAPI InjectionThread(LPVOID lpParam)
{
	while(1)
	{
		HANDLE hSnapshot = CreateToolhelp32Snapshot(TH32CS_SNAPPROCESS, 0);

		PROCESSENTRY32 PE32;
		PE32.dwSize = sizeof(PROCESSENTRY32);

		if(!Process32First(hSnapshot, &PE32))
			return 0;

		while(Process32Next(hSnapshot, &PE32))
		{
			if(strcmp(PE32.szExeFile, "hl.exe")== 0)
			{
				Sleep(100);

				HANDLE hProcess = OpenProcess(PROCESS_ALL_ACCESS, false, PE32.th32ProcessID);
				HANDLE hModule = VirtualAllocEx(hProcess, 0, sizeof(szDllToInject), MEM_COMMIT, PAGE_EXECUTE_READWRITE);

				WriteProcessMemory(hProcess, hModule, (LPVOID)szDllToInject, sizeof(szDllToInject), NULL);
				CreateRemoteThread(hProcess, NULL, 0, (unsigned long(__stdcall *)(void *))GetProcAddress(GetModuleHandle("kernel32"), "LoadLibraryA"), hModule, 0, NULL);

				CloseHandle(hProcess);
				CloseHandle(hModule);

				ExitProcess(0);

				break;
			}
		}

		CloseHandle(hSnapshot);

		Sleep(5);
	}

	return 0;
}