一个小问题,XP改成vista系统后,然后运行的时候,验证码显示不出来啦,下面是异常报告.....

为什么我用vista以后，我的网页就显示不了验证码啦？？！！ 在XP里就可以正常显示...
严重: Servlet.service() for servlet imgNum threw exception 
java.lang.NoClassDefFoundError: Could not initialize class javax.imageio.ImageTypeSpecifier 
at javax.imageio.ImageIO.write(ImageIO.java:1467) 
at javax.imageio.ImageIO.write(ImageIO.java:1565) 
at web.RandomImgCodeServlet.doGet(RandomImgCodeServlet.java:41) 
at javax.servlet.http.HttpServlet.service(HttpServlet.java:690) 
at javax.servlet.http.HttpServlet.service(HttpServlet.java:803) 
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290) 
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206) 
at agent.view.util.SetCharacterEncodingFilter.doFilter(SetCharacterEncodingFilter.java:40) 
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235) 
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206) 
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:233) 
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:175) 
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:128) 
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102) 
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109) 
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:286) 
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:844) 
at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:583) 
at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:447) 
at java.lang.Thread.run(Thread.java:619)