将以下代码放到一个单独的页面checkcode.asp
<%
Option Explicit
Response.buffer=true
Call Com_CreatValidCode("validateCode")

Sub Com_CreatValidCode(pSN)
\' 禁止缓存
Response.Expires = -9999
Response.AddHeader "Pragma","no-cache"
Response.AddHeader "cache-ctrol","no-cache"
Response.C

Randomize

Dim i, ii, iii

Const cOdds = 2 \' 杂点出现的机率
Const cAmount = 10 \' 文字数量
Const cCode = "0123456789"

\' 颜色的数据(字符，背景)
Dim vColorData(1)
vColorData(0) = ChrB(0) & ChrB(0) & ChrB(211) \' 蓝0，绿0，红0（黑色）
vColorData(1) = ChrB(255) & ChrB(255) & ChrB(255) \' 蓝250，绿236，红211（浅蓝色）

\' 随机产生字符
Dim vCode(4), vCodes
For i = 0 To 3
vCode(i) = Int(Rnd * cAmount)
vCodes = vCodes & Mid(cCode, vCode(i) + 1, 1)
Next
Session(pSN) = vCodes \'记录入Session
\' 字符的数据
Dim vNumberData(9)
vNumberData(0) = "1110000111110111101111011110111101001011110100101111010010111101001011110111101111011110111110000111"
vNumberData(1) = "1111011111110001111111110111111111011111111101111111110111111111011111111101111111110111111100000111"
vNumberData(2) = "1110000111110111101111011110111111111011111111011111111011111111011111111011111111011110111100000011"
vNumberData(3) = "1110000111110111101111011110111111110111111100111111111101111111111011110111101111011110111110000111"
vNumberData(4) = "1111101111111110111111110011111110101111110110111111011011111100000011111110111111111011111111000011"
vNumberData(5) = "1100000011110111111111011111111101000111110011101111111110111111111011110111101111011110111110000111"
vNumberData(6) = "1111000111111011101111011111111101111111110100011111001110111101111011110111101111011110111110000111"
vNumberData(7) = "1100000011110111011111011101111111101111111110111111110111111111011111111101111111110111111111011111"
vNumberData(8) = "1110000111110111101111011110111101111011111000011111101101111101111011110111101111011110111110000111"
vNumberData(9) = "1110001111110111011111011110111101111011110111001111100010111111111011111111101111011101111110001111"
\' 输出图像文件头
Response.BinaryWrite ChrB(66) & ChrB(77) & ChrB(230) & ChrB(4) & ChrB(0) & ChrB(0) & ChrB(0) & ChrB(0) &_
ChrB(0) & ChrB(0) & ChrB(54) & ChrB(0) & ChrB(0) & ChrB(0) & ChrB(40) & ChrB(0) &_
ChrB(0) & ChrB(0) & ChrB(40) & ChrB(0) & ChrB(0) & ChrB(0) & ChrB(10) & ChrB(0) &_
ChrB(0) & ChrB(0) & ChrB(1) & ChrB(0)

\' 输出图像信息头
Response.BinaryWrite ChrB(24) & ChrB(0) & ChrB(0) & ChrB(0) & ChrB(0) & ChrB(0) & ChrB(176) & ChrB(4) &_
ChrB(0) & ChrB(0) & ChrB(18) & ChrB(11) & ChrB(0) & ChrB(0) & ChrB(18) & ChrB(11) &_
ChrB(0) & ChrB(0) & ChrB(0) & ChrB(0) & ChrB(0) & ChrB(0) & ChrB(0) & ChrB(0) &_
ChrB(0) & ChrB(0)

For i = 9 To 0 Step -1 \' 历经所有行
For ii = 0 To 3 \' 历经所有字
For iii = 1 To 10 \' 历经所有像素
\' 逐行、逐字、逐像素地输出图像数据
If Rnd * 99 + 1 < cOdds Then \' 随机生成杂点
Response.BinaryWrite vColorData(0)
Else
Response.BinaryWrite vColorData(Mid(vNumberData(vCode(ii)), i * 10 + iii, 1))
End If
Next
Next
Next
End Sub
%>

需要用得页面用如下方式引用：
验证码： <input name="txt_check" type="text" size=6 maxlength=4 class="input"> <img src="checkcode.asp " alt="验证码,看不清楚?请点击刷新验证码" height="10" style="cursor : pointer;" >

判断页面：
<%
if trim(session("validateCode")) <> trim(Request("txt_check")) then
ErrorMessage = "请输入正确的验证码"
response.write(" <script>alert(\'"&ErrorMessage&"\');location.href=\'Default.asp\' </script>")
response.end
end if
%>