问题解决了。解决过程如下：
首先在网上找到了一遍关于这个补丁不能安装的文章：
“最近用MBSA扫描系统的时候发现有新的升级没有安装,于是就把那些升级文件下载下来安装.其中编号为KB896688的关键性升级老是不成功.总是显示KB896688 安装程序被取消。也不说明是为什么.百思不得其解的时候,想到了在windows文件夹下应该有log文件的.打开之后发现Unexpected Error While Executing Line 1 ( Test.IE7InstallBlock.Section ) of PreRequisite 有这么个错误.看样子好像是要测试IE7的InstallBlock,但也没有说为什么会不成功.实在是没有办法了,就用Test.IE7InstallBlock.Section为关键字,到google搜了一下,只找到一个网页,原文如下f you have changed your Program Files directory after installing windows,
and before trying to apply the 896688 critical update, the update's version
check will fail as it will only check the new Program Files directory that
you have designated when your iexplore.exe is probably in C:\Program Files\.
To fix this, copy all the files missing from your new Program Files
directory from the C:\Program Files\ directory, apply the critical update,
reboot, and then copy all the updated files from the new Program Files
directory into the C:\Program Files\ directory, replacing all old files.
大意是说如果改变的windows默认的文件安装文件夹,就会出现这种情况,刚好和我的情况相符合,我把默认的文件夹改到D盘了.后面的跟贴也有说明An alternative to copying the files is to reset the registry key for your
Program Files back to C:\Program Files while you install the update, then set
it back to your normal value. This worked for me.
The registry key is:
HKEY_LOCAL_MACHINE
SOFTWARE
Microsoft
Windows
CurrentVersion
ProgramFilesDir
就是说改回来就可以了.于是改回原来的值,这次是终于可以安装了.”
不过我的并没有改变默认安装路径，只是从中得到启发，想起以前我将IE禁用了（避免IE自动弹出广告），恢复正常权限，就可以了。
有这种问题的兄弟们，多朝着这方面思考一下，一定会有所启发的。
