Tomcat4.1与Sqlserver 连接问题

机器配置: Win2k server+ Sqlserver 2K + Tomcat 4.1+ j2sdk1.4.1+ Microsoft JDBC
Server.xml配置如下:
<Context path="IDsoft" docBase="/IDsoft" reloadable="true">
<ResourceParams name="jdbc/dbcon">
<parameter><name>user</name><value>sa</value></parameter>
<parameter><name>password</name><value></value></parameter>
<parameter><name>driverClassName</name>
<value>com.microsoft.jdbc.sqlserver.SQLServerDriver</value></parameter>
<parameter><name>driverName</name>
<value>jdbc:microsoft:sqlserver://192.168.0.191:1433;DatabaseName=school</value></parameter>
</ResourceParams>
</Context>

web.xml:
<Resource-ref>
<res-ref-name>jdbc/dbcon</res-ref-name>
<res-type>javax.sql.DataSource</res-type>
<res-auth>Container</res-auth>
</resource-ref>
//test.jsp
<%@ page import="java.sql.*"%>
<%@ page import="javax.naming.Context"%>
<%@ page import="javax.sql.DataSource"%>
<%@ page import="javax.naming.InitialContext"%>
<%
DataSource ds = null;
Connection con=null;
try{
Context initCtx = new InitialContext();
Context envCtx = (Context) initCtx.lookup("java:comp/env");
ds = (DataSource)envCtx.lookup("jdbc/dbcon");
con = ds.getConnection();
Statement stmt=con.createStatement();
}catch(Exception ex){
out.println(ex.getMessage());
}
%>

ex抛出提示: Cann't load JDBC driver class 'null';请问是哪儿配置错了?