I had a similar problem on a Windows 7 virtual machine. I did some investigation and found that Windows Update service was causing svchost.exe to use all the CPU, and it would never find an update.
I eventually found the solution here
https://support.microsoft.com/en-us/kb/3102810You can't find this update because Windows Update service is broken. You need to stop the service, manually download the fix and install it, then reboot.
After doing that, and checking for updates again, I found another 238 updates available (1GB).