I've had an issue a couple of times now when testing/running Windows 8 in an environment where group policy enforces WSUS settings causing a problem as the WSUS server hasn't been updated to support Windows 8 clients or hasn't had the required updates published.
Sometimes this can easily be got around by simply clicking the "Check online for updates from Windows Update" link, however this was removed by GPO in these cases.
In these cases I've had to fall back to using this simple batch file script:
REG ADD "HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows\WindowsUpdate\AU" /v UseWUServer /t REG_DWORD /d 0 /f
net stop "Windows Update"
net start "Windows Update"
control /name Microsoft.WindowsUpdate
After running this batch file, you can run Windows Update as normal and it will go directly to Windows Updates to check.
This is obviously not something a normal "user" should use, but then they shouldn't have the access rights to run this anyway! Neither is it a permanent workaround, just a quick fix while the WSUS server is updated to provide Windows 8 updates.
Previous blog post on preparing WSUS for Windows 8/Server 2012: