#ifdef UNICODE
#define LoadLibraryTCHAR  "LoadLibraryW"
#else
#define LoadLibraryTCHAR  "LoadLibraryA"
#endif

typedef HINSTANCE (*fpLoadLibrary)(TCHAR*);
typedef LPVOID (*fpGetProcAddress)(HINSTANCE, TCHAR*);
typedef void (*fpFunktion)(void);

struct INJECTSTRUCT
{
      fpLoadLibrary LoadLibrary;
      fpGetProcAddress GetProcAddress;
      TCHAR path[255];
      TCHAR func[255];
};

DWORD WINAPI threadstart(LPVOID addr)
{
	HINSTANCE hDll;
	fpFunktion funktion;
	INJECTSTRUCT * is = (INJECTSTRUCT*)addr;       
	hDll = is->LoadLibrary(is->path);
	funktion = (fpFunktion)is->GetProcAddress(hDll, is->func);
	funktion();
	return 0;
}

void threadend()
{
}

bool EnableDebugPrivilege()
{
	TOKEN_PRIVILEGES priv;
	HANDLE hThis, hToken;
	LUID luid;
	hThis = GetCurrentProcess();
	OpenProcessToken(hThis, TOKEN_ADJUST_PRIVILEGES, &hToken);
	LookupPrivilegeValue(0, TEXT("seDebugPrivilege"), &luid);
	priv.PrivilegeCount = 1;
	priv.Privileges[0].Luid = luid;
	priv.Privileges[0].Attributes = SE_PRIVILEGE_ENABLED;
	AdjustTokenPrivileges(hToken, false, &priv, 0, 0, 0);
	CloseHandle(hToken);
	CloseHandle(hThis);
	return true;
}

HANDLE InjectDll(unsigned ProcessId, TCHAR *DllFile, TCHAR* FunctionName)
{
	if(ExistFile(DllFile) == false)
	{
		MessageBox(0, TEXT("Can't find the DLL file!"), szAppName, MB_ICONERROR);
		return 0;
	}

	INJECTSTRUCT is;
	_tcscpy_s(is.path, DllFile);
	_tcscpy_s(is.func, FunctionName);
	DWORD funcsize = (DWORD)threadend - (DWORD)threadstart;

	EnableDebugPrivilege();

	HINSTANCE hDll = LoadLibrary(TEXT("KERNEL32"));
	if(hDll == NULL)
	{
		MessageBox(0, TEXT("LoadLibrary failed!"), szAppName, MB_ICONERROR);
		return 0;
	}

	is.LoadLibrary = (fpLoadLibrary)GetProcAddress(hDll, LoadLibraryTCHAR);
	is.GetProcAddress = (fpGetProcAddress)GetProcAddress(hDll, "GetProcAddress");
	if(is.LoadLibrary == NULL || is.GetProcAddress == NULL)
	{
		MessageBox(0, TEXT("GetProcAddress failed!"), szAppName, MB_ICONERROR);
		return 0;
	}

	HANDLE hProc = OpenProcess(PROCESS_ALL_ACCESS, false, ProcessId);
	if(hProc == NULL)
	{
		MessageBox(0, TEXT("OpenProcess failed!"), szAppName, MB_ICONERROR);
		return 0;
	}

	LPVOID start = VirtualAllocEx(hProc, 0, funcsize+sizeof(INJECTSTRUCT), MEM_COMMIT, PAGE_EXECUTE_READWRITE);
	if(start == NULL)
	{
		MessageBox(0, TEXT("VirtualAllocEx failed!"), szAppName, MB_ICONERROR);
		return 0;
	}

	if(WriteProcessMemory(hProc, start, (LPVOID)&is, sizeof(INJECTSTRUCT), NULL) == 0)
	{
		MessageBox(0, TEXT("WriteProcessMemory failed!"), szAppName, MB_ICONERROR);
		return 0;
	}

	LPVOID thread = (LPVOID)((DWORD)start+sizeof(INJECTSTRUCT));

	if(WriteProcessMemory(hProc, thread, (LPVOID)threadstart, funcsize, NULL) == 0)
	{
		MessageBox(0, TEXT("WriteProcessMemory failed!"), szAppName, MB_ICONERROR);
		return 0;
	}

	HANDLE threadID = CreateRemoteThread(hProc, 0, 0, (LPTHREAD_START_ROUTINE)thread, start, 0, 0);
	if(threadID == NULL)
	{
		MessageBox(0, TEXT("CreateRemoteThread failed!"), szAppName, MB_ICONERROR);
		return 0;
	}

	if(CloseHandle(hProc) == 0)
	{
		MessageBox(0, TEXT("CloseHandle failed!"), szAppName, MB_ICONERROR);
		return 0;
	}

	return threadID;
}

Würde mich wahnsinnig freuen, wenn jemand weiß, warum das als Virus erkannt wird oder was ich ändern muss.

Kleiner Tipp:
http://msdn.microsoft.com/en-us/library/ms683212

die ist anscheinend nicht falsch. Es geht ja in ASCII. Das Problem ist nur UNICODE

Na, wenn Du das sagst, dann wird das schon stimmen...

Also gut, ich geb Dir noch ein Tipp: Der 2. Parameter ist falsch deklariert...

typedef LPVOID (*fpGetProcAddress)(HMODULE , TCHAR*); 

msdn:
FARPROC WINAPI GetProcAddress(
  __in  HMODULE hModule,
  __in  LPCSTR lpProcName // pointer zu char array und da unicode TCHAR.
);

etwas "wärmer" wirds, wenn Du dort suchst, wo der zweite Parameter als fixen Text-String an die besagte Funktion übergeben wird (kommt nur einmal in Deinem Code vor)

na, dann müßtest Du drauf kommen wo der Unicode-Fehler steckt!
Martin

Und auch die falsche definition des "func" Feldes in der Struktur...

typedef HINSTANCE (__stdcall *fpLoadLibrary)   (TCHAR*);
typedef LPVOID    (__stdcall *fpGetProcAddress)(HINSTANCE, LPCSTR);  
typedef void (*fpFunktion)(void);

struct INJECTSTRUCT
{
      fpLoadLibrary LoadLibrary;
      fpGetProcAddress GetProcAddress;
      TCHAR path[255];
      LPCSTR func;
};

DWORD WINAPI threadstart(LPVOID addr)
{
    HINSTANCE hDll;
    fpFunktion funktion;
    INJECTSTRUCT * is = (INJECTSTRUCT*)addr;      
    hDll = is->LoadLibrary(is->path);
    funktion = (fpFunktion)is->GetProcAddress(hDll, is->func);
    funktion();
    return 0;
}

void threadend()
{
}

bool EnableDebugPrivilege()
{
    TOKEN_PRIVILEGES priv;
    HANDLE hThis, hToken;
    LUID luid;
    hThis = GetCurrentProcess();
    OpenProcessToken(hThis, TOKEN_ADJUST_PRIVILEGES, &hToken);
    LookupPrivilegeValue(0, TEXT("seDebugPrivilege"), &luid);
    priv.PrivilegeCount = 1;
    priv.Privileges[0].Luid = luid;
    priv.Privileges[0].Attributes = SE_PRIVILEGE_ENABLED;
    AdjustTokenPrivileges(hToken, false, &priv, 0, 0, 0);
    CloseHandle(hToken);
    CloseHandle(hThis);
    return true;
}

HANDLE InjectDll(unsigned ProcessId, TCHAR *DllFile, LPCSTR FunctionName)
{
    if(ExistFile(DllFile) == false)
    {
        MessageBox(0, TEXT("Can't find the DLL file!"), szAppName, MB_ICONERROR);
        return 0;
    }

    INJECTSTRUCT is;
    _tcscpy_s(is.path, DllFile);
    is.func = FunctionName;
    DWORD funcsize = (DWORD)threadend - (DWORD)threadstart;

    EnableDebugPrivilege();

    HINSTANCE hDll = LoadLibrary(TEXT("KERNEL32"));
    if(hDll == NULL)
    {
        MessageBox(0, TEXT("LoadLibrary failed!"), szAppName, MB_ICONERROR);
        return 0;
    }

    is.LoadLibrary = (fpLoadLibrary)GetProcAddress(hDll, "LoadLibraryA");
    is.GetProcAddress = (fpGetProcAddress)GetProcAddress(hDll, "GetProcAddress");
    if(is.LoadLibrary == NULL || is.GetProcAddress == NULL)
    {
        MessageBox(0, TEXT("GetProcAddress failed!"), szAppName, MB_ICONERROR);
        return 0;
    }

    HANDLE hProc = OpenProcess(PROCESS_ALL_ACCESS, false, ProcessId);
    if(hProc == NULL)
    {
        MessageBox(0, TEXT("OpenProcess failed!"), szAppName, MB_ICONERROR);
        return 0;
    }

    LPVOID start = VirtualAllocEx(hProc, 0, funcsize+sizeof(INJECTSTRUCT), MEM_COMMIT, PAGE_EXECUTE_READWRITE);
    if(start == NULL)
    {
        MessageBox(0, TEXT("VirtualAllocEx failed!"), szAppName, MB_ICONERROR);
        return 0;
    }

    if(WriteProcessMemory(hProc, start, (LPVOID)&is, sizeof(INJECTSTRUCT), NULL) == 0)
    {
        MessageBox(0, TEXT("WriteProcessMemory failed!"), szAppName, MB_ICONERROR);
        return 0;
    }

    LPVOID thread = (LPVOID)((DWORD)start+sizeof(INJECTSTRUCT));

    if(WriteProcessMemory(hProc, thread, (LPVOID)threadstart, funcsize, NULL) == 0)
    {
        MessageBox(0, TEXT("WriteProcessMemory failed!"), szAppName, MB_ICONERROR);
        return 0;
    }

    HANDLE threadID = CreateRemoteThread(hProc, 0, 0, (LPTHREAD_START_ROUTINE)thread, start, 0, 0);
    if(threadID == NULL)
    {
        MessageBox(0, TEXT("CreateRemoteThread failed!"), szAppName, MB_ICONERROR);
        return 0;
    }

    if(CloseHandle(hProc) == 0)
    {
        MessageBox(0, TEXT("CloseHandle failed!"), szAppName, MB_ICONERROR);
        return 0;
    }

    return threadID;
}

struct INJECTSTRUCT
{
      fpLoadLibrary LoadLibrary;
      fpGetProcAddress GetProcAddress;
      TCHAR path[255];
      char func[255];
};

HANDLE InjectDll(unsigned ProcessId, TCHAR *DllFile, LPCSTR FunctionName)
{
    if(ExistFile(DllFile) == false)
    {
        MessageBox(0, TEXT("Can't find the DLL file!"), szAppName, MB_ICONERROR);
        return 0;
    }

    INJECTSTRUCT is;
    _tcscpy_s(is.path, DllFile);
    strcpy_s(is.func, FunctionName);
    DWORD funcsize = (DWORD)threadend - (DWORD)threadstart;
    [...]

Was denn?
Du nimmst Visual Studio her, stellst in den general project properties all configurations auf multibyte character set und verwendest dann kein L oder _T oder sonstigen unnötigen Mist sondern schreibst einfach den Text.

Und dann geht's mit UNICODE?