Try clicking "Validate Now" over on the right-hand side...http://www.microsoft.com/genuine/default.aspx?displaylang=en&PartnerID=4
If the validation tool for whatever reason fails to install after the download, WindowsUpdate will fail with no error messages what-so-ever.
What does the WindowsUpdate.Log, SetupAPI.Log file under Windows directory state ?
This was a case unique to some changes that I had made on my system myself... but I had changed the Temp directories to custom locations.
There are temp directories spread throughout the system and thought it would be more efficient to monitor and clear them if I concatenated them by changing the environment variables for "HKEY_CURRENT_USER\Environment" and "HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\Environment" --and-- using NTFS Junctions to redirect their physical locations.
There are three potential problems with this.
1) Permissions on the new locations must be modified to accept all the appropriate users and accounts.
2) Different users and accounts may create files that clash with one-another.
3) Windows Update and possibly related services appear to be incompatible with NTFS Junctions.
Even when the first two issues were resolved, the third... remained an impass.
So essentially, if there are any anomolies (permissions or otherwise) with "LocalService\Local Settings\Temp", "USERPROFILE\Local Settings\Temp", "Windows\System32\Temp", "LocalService\Local Settings\Temporary Internet Files" and "USERPROFILE\Local Settings\Temporary Internet Files", "Windows\Downloaded Program Files" and "Windows\SoftwareDistribution"... it may not work.
May also fail if validation for whatever reason(s) fail to install or run properly.