出错原因：
当用window.showModalDialog打开模态窗口时，模态窗口中的页面访问了 "navigator.userAgent"。
访问navigator.userAgent出错的原因和解决方法：
IE的User-Agent键下含非标准值,超出默认的字符空间,以致于出错.
可以查找注册表中的以下位置，看是否存在过长的字符串。
"HKCU\Software\Microsoft\Windows\CurrentVersion\Internet Settings\5.0\User Agent\Post Platform"
"HKCU\Software\Microsoft\Windows\CurrentVersion\Internet Settings\User Agent\Post Platform"
"HKLM\Software\Microsoft\Windows\CurrentVersion\Internet Settings\5.0\User Agent\Post Platform"
"HKLM\Software\Microsoft\Windows\CurrentVersion\Internet Settings\User Agent\Post Platform"
"HKCU\Software\Microsoft\Windows\CurrentVersion\Internet Settings\User Agent\Post Platform"
"HKLM\Software\Microsoft\Windows\CurrentVersion\Internet Settings\5.0\User Agent\Post Platform"
"HKLM\Software\Microsoft\Windows\CurrentVersion\Internet Settings\User Agent\Post Platform"
相关资料：
http://www.blogjava.net/emu/archive/2007/08/24/139182.htmlhttp://www.cnblogs.com/leonny/archive/2009/04/28/1445727.html
http://hi.baidu.com/btx_1210/blog/item/1c9c691fc5ddadf0e1fe0b09.html