How to "reset" windows update

I've been facing a problem which took me several months to discover. Since October 2008, WinUpdate is not installing the Vista Professional security - all other updates are succefull but not the one related to Vista Security update. I was recommended to delete the content of \windows\softwaredistribution directory which i did and the installation of updates dated of January 2009 we successfull. Now simple question, is it possible to have WU install the previous update in order to catch up with missing updates ? How is that feasable, if indeed feasable ? Best regards and thanks in advance for the help

all latest updates will contain all previous updates within them, confused? not to worry I shall explain, if updates didnt contain previous ones then when you re-install vista, it wont download about 500+ updates that have been released since vista's release, its not cost effective and efficient for microsoft or the consumer, putting it simply, by not downloading previous updates you are infact being efficient.