Hello and welcome to the Vista Forums!
The trick for this error lies in pending.xml. What we need to do is delete C:\Windows\winsxs\pending.xml.
EDIT: Actually, back it up first. Please rename it to pen-ding.bak. Destroy the name pending as well. Then do the rest of this post. Please be warned that this may corrupt the update. Please stick with me afterwards to try to stop it happening with every update.
Do you have a disk yet? Make one here: Create a Recovery Disc
Follow this tutorial: System Recovery Options
Try a System Restore. If that doesn't do it, boot into a Command Prompt from that CD. Type notepad
and press enter. Notepad > File > Open
In the dialogue that comes up, you have a mini explorer. Navigate to C:\Windows\winsxs\ and delete pending.xml.
Try to boot your computer. See a very comprehensive post here if necessary: stuck in update mode - Windows 7 Forums
The very best of luck!