I fixed this issue for a customer this morning on a Windows 7 machine. When you loaded Windows Update, it’d say no updates are available despite the fact the machine had no service pack installed (SP1 is the latest release at the time of writing).
I tried to install the System Update Readiness Tool but was presented with the error “Installer encountered an error: 0xc8000222”.
The fault was caused by a corrupted SoftwareDistribution folder. So if you rename or delete %windir%\SoftwareDistribution and then try Windows Update or the System Update Readiness Tool, the problem is then fixed.
Supposedly the same issue can occur on Windows Vista and would have the same fix.