This so annoying ARRGGG

one teeny, tiny update is stoping me from installing SP1 (i want the stability dammit) but it won't install and i don't know what to do.
The update is:
Hidden content (Click to reveal)Update for Windows Vista for x64-based Systems (KB935509)
Download size: 551 KB
You may need to restart your computer for this update to take effect.
Update type: Important
Install this update to enable future updates to install successfully on Windows Vista Ultimate and Windows Vista Enterprise. This update may be required before selected future updates can be installed. After you install this item, you may have to restart your computer. Once you have installed this item, it cannot be removed.
More information:
http://go.microsoft.com/fwlink/?LinkId=105795Help and Support:
http://support.microsoft.com the error code i get is :80073715
I can't tell you much more but i really need to know what i can do to fix it, since its:
a) stoping me from updateing
b) anoying me since it means somthing is wrong and i don't like it
so please, help me?
EDIT: i notice that when i download this update in microsoft update, it gets stuck at 7% before it tells me it failed