I've had a simular problem with windows update for the last few months.
so I stumbled on a fix that works for me (and no, I have no idea why it works).
when you run the unpdate page it shows you driver updates, xp updates, and critical updates (with the critical updates automaticly selected for download). when I'd select 'download and install' it would run and give me the error. so anyway one day I was just staring at it all pissed off and bored and cancelled the selection of the critical update (leaving noe selected) then rechose it and ran the download. everything worked fine.
so far I've had the error on the last 4 critical updates and this has fixed every one.
I don't understand it, it just works for me.