I had the same problem with Windows Update and Microsoft Security Essentials (update files would not install), the cause undoubtedly being one program from the latest series up updates for my motherboard (this was not a problem for the files on my motherboard's
DVD). I found a solution on the Web, buried somewhere in these forums:
For Windows 7:
1. Go to the 'Start' menu and type 'services.msc', then run the program.
2. Look for 'WindowsUpdate' in services.msc, right-click and choose 'Stop'.
3. Go to C:\Windows\SoftwareDistribution and delete everything in that folder.
[Not part of the procedure, but I chose to restart the machine at this point.]
4. Go back to services.msc, find 'WindowsUpdate', right-click and choose 'Start'.
If you have MSE installed and you experienced trouble, you might do well to remove it and reinstall a fresh copy, before reactivating the Web to download updates.