The Windows 7 update "System Update Readiness Tool for Windows 7 (KB947821) [May 2014]" will appear hung or stuck during the update process. It will get to about 80 to 90% and just the hard drive activity is all that will indicate that it is still working on the update process. Many have noted it to take several hours. In our own install of this for a client it took almost 6 hours to complete on a system that failed the SFC scan test.
If you are installing the System Update Readiness Tool for Windows 7 (KB947821) besure to allow for a long period of time to install. Do not stop this install or you may corrupt the system to a point where it will not boot and you will be required to reload Windows from scratch.
Although we have not tested this yet, this tool might be an option to run if you have a system that has not passed the SFC test due to file corruption.
We hope that this information is helpful. Please let us know how this has helped you or if you have additional questions. As always Firestone Technical Resources, Inc. is here to help with your computer support issues - "Providing personal service for your impersonal technology."