using System;
using System.Collections.Generic;
using
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;

namespace WindowsApplication2
{
    public partial class Form1 : Form
    {
        int x,y,n;
        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            if (textBox1.Text.Trim() == "石头")
            { y = 1; }
            else
            {
                if (textBox1.Text.Trim() == "剪刀")
                {y=2; }
                else
                {
                    if (textBox1.Text.Trim() == "步")
                    { y = 3; }
                    else { MessageBox.Show("输入错误！"); }
                }
            }
            Random m = new Random();
            x = m.Next(3);
            if (x == 1) { label1.Text = "石头"; }
            else { if (x == 2) { label1.Text = "剪刀"; } else { label1.Text = "布"; } }
            if(x==1&&y==1||x==2&&y==2||x==3&&y==3){MessageBox.Show("平局！");}
            else
            {
                if(x==1&&y==2||x==2&&y==3||x==3&&y==1){MessageBox.Show("您输了！");}
                else{MessageBox.Show("您赢了！");n++;}
            }
            if(n==3){MessageBox.Show("本局您获胜！");n=0;}

        }
    }
}   

把继承自form改成继承自system.web.ui.page
然后messagebox.show换成Response.write("<script>alert('提示信息');</script>");