Had this myself. Bit of corruption in its list of updates, apparently.
1) Go to Administrative Tools | Services, and stop the Windows Update service.
2) Go to the Windows folder. Rename the folder called SoftwareDistribution to SoftwareDistribution-old. [or whatever]
3) Go back to Administrative Tools | Services, and start the Windows Update service.
4) Then try another Windows Update.
Should now work fine; after which you can delete the old folder, cos it will have built a new working one!