要设置快捷键必须使用user32.dll下面的两个方法。 
BOOL RegisterHotKey( //注册系统热键的API函数
　HWND hWnd, 
　int id, 
　UINT fsModifiers, 
　UINT vk 
); 

BOOL UnregisterHotKey( //删除系统热键的API函数
　HWND hWnd, 
　int id 
); 

在C#中引用命名空间System.Runtime.InteropServices;来加载非托管类user32.dll
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;

namespace HotKey
{

    public enum KeyModifiers //组合键枚举
    {
        None = 0,
        Alt = 1,
        Control = 2,
        Shift = 4,
        Windows = 8
    } 

    public partial class Form1 : Form
    {
        /*
         * RegisterHotKey函数原型及说明：
         * BOOL RegisterHotKey(
         * HWND hWnd,         // window to receive hot-key notification
         * int id,            // identifier of hot key
         * UINT fsModifiers, // key-modifier flags
         * UINT vk            // virtual-key code);
         * 参数 id为你自己定义的一个ID值
         * 对一个线程来讲其值必需在0x0000 - 0xBFFF范围之内,十进制为0~49151
         * 对DLL来讲其值必需在0xC000 - 0xFFFF 范围之内,十进制为49152~65535
         * 在同一进程内该值必须唯一参数 fsModifiers指明与热键联合使用按键
         * 可取值为：MOD_ALT MOD_CONTROL MOD_WIN MOD_SHIFT参数，或数字0为无，1为Alt,2为Control，4为Shift，8为Windows
         * vk指明热键的虚拟键码
         */

        [System.Runtime.InteropServices.DllImport("user32.dll")] //申明API函数
        public static extern bool RegisterHotKey(
         IntPtr hWnd, // handle to window 
         int id, // hot key identifier 
         uint fsModifiers, // key-modifier options 
         Keys vk // virtual-key code 
        );
        [System.Runtime.InteropServices.DllImport("user32.dll")] //申明API函数
        public static extern bool UnregisterHotKey(
         IntPtr hWnd, // handle to window 
         int id // hot key identifier 
        );
        public Form1()
        {
            InitializeComponent();
        }
        private void ProcessHotkey(Message m) //按下设定的键时调用该函数
        {
            IntPtr //IntPtr用于表示指针或句柄的平台特定类型
            //MessageBox.Show(id.ToString());
            string sid = id.ToString();
            switch (sid)
            {
                case "100":
                    MessageBox.Show("调用A函数");
                    break;
                case "200":
                    MessageBox.Show("调用B函数");
                    break;
            }
        }
        private void Form1_Load(object sender, EventArgs e)
        {
            //Handle为当前窗口的句柄,继续自Control.Handle,Control为定义控件的基类
            //RegisterHotKey(Handle, 100, 0, Keys.A); //注册快捷键,热键为A
            //RegisterHotKey(Handle, 100, KeyModifiers.Alt | KeyModifiers.Control, Keys.B);//这时热键为Alt+CTRL+B
            //RegisterHotKey(Handle, 100, 1, Keys.B); //1为Alt键，热键为Alt+B
            RegisterHotKey(Handle, 100, 2,Keys.A); //定义热键为Alt+Tab，这里实现了屏幕系统Alt+Tab键
            RegisterHotKey(Handle, 200, 2, Keys.B); //注册2个热键,根据id值100,200来判断需要执行哪个函数
        }

        private void button1_Click(object sender, EventArgs e) //重新设置热键
        {
            UnregisterHotKey(Handle, 100);//卸载快捷键
            RegisterHotKey(Handle, 100, 2, Keys.C); //注册新的快捷键，参数0表示无组合键
        }

        private void Form1_FormClosing(object sender, FormClosingEventArgs e) //退出程序时缷载热键
        {
            UnregisterHotKey(Handle, 100);//卸载第1个快捷键
            UnregisterHotKey(Handle, 200); //缷载第2个快捷键
        }

        //重写WndProc()方法，通过监视系统消息，来调用过程
        protected override void WndProc(ref Message m)//监视Windows消息
        {
            const int WM_HOTKEY = 0x0312;//如果m.Msg的值为0x0312那么表示用户按下了热键
            switch (m.Msg)
            {
                case WM_HOTKEY:
                    ProcessHotkey(m);//按下热键时调用ProcessHotkey()函数
                    break;
            }
            base.WndProc(ref m); //将系统消息传递自父类的WndProc
        }
    }
}