check if you have enough free disk space (at least 1 gigabyte) on your hard drive. If not, read the Remove temporary files article on how to get rid of unnecessary files with CCleaner.
verify that time and date are correct on your PC - a date in the past or the future prevents both Windows Update and Genuine Windows Validation tool from working properly.
run a disk check to verify that file system is not corrupt. Right click start - command prompt (admin) - type sfc /scannow - when it completes type exit to come out of command prompt
try a restart rather that a shutdown this can often fix problems with windows.
Retry an update anbd see if it will now go in