Can you try these things:
Reboot. Then try again. (The reason behind this is that previously applied windows updates might block other windows updates until the host has restarted.
Check for error message in
WindowsUpdate.log (This file is usually found in c:\windows). Then check the event logs
If it has a error, act on it, if not go to 2)
Log in with an administrative account.
[start] [run] [cmd]
net stop wuauserv
(leave the prompt open)
Go to %windir&\SoftwareDistribution (Usually
Delete everything. (You can do this because the windows update service is stopped).
Go back to the open cmd prompt and enter
net start wuauserv.
(Some files will not reappear in the SoftwareDistribution folder.)
Try windows update again.
If it still fails go to another computer and download a liveCD.
Boot from the CD and check the computer for a virus and or a root kit. (Some of these block windows updates).
This post on SuperUser has a lot of information in this.