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.
A client had an HP P2-1394 model H3Z88AA desktop system that would turn on briefly, the drives would power up, the fans would spin, and then the system would shut down. After about 5 seconds the process would repeat. This would continue until either the power was removed or the power button was pressed for several seconds forcing the system to the off mode.
Some technical notes found on similar issues suggested the following: