I have had problems with MS Updates when they were blocked by Norton's. You may want to try it with a disabled AV.
Apart from that, as I recall there are two ways to find out the reason why an update failed to install.
1) Using Internet Explorer, go to update.microsoft.com
. On the left you should have an option for update history
(or similar). Clicking on that will take you to a screen showing a list of updates. There should be a question mark for those that are failed, which you can click. This will give you a little more information.
2) The longer, more involved (but more detailed) method is to go straight to the update log. Click Start
and type/paste %windir%\windowsupdate.log
in the textbox. Its usually pretty hard to interpret unless you are used to it, but look for words like warning
For an official MS article on the issue, you can read it here:http://support.microsoft.com/kb/906602
Alot of this post is from memory, so if there are innacuracies my apologies.