Windows7 an old machine, power-on reset has been stuck in the Windows Update 35?oing round in circles for hours, unable to enter the system.
forcibly press the OFF button, F8 to enter safe mode remains the same phenomenon. After
access to MSDN, it was mentioned by DISM undo the last system operation can solve the problem.
with WinRE WinPE recovery mode or start, run the following command, where X: is the letter of Win7 system is located.

DISM /Image:X:\ /Cleanup-Image /RevertPendingActions

restart, the problem has been resolve, directly into the Win7 login screen.

DISM Introduction

DISM (Deployment Image Servicing and Management) is the Deployment Image Servicing and Management tool (DISM.exe) used to install, uninstall, configure, and update an offline Windows (R ) images and offline pre-installed Windows environment (Windows PE) functions and packages in the image. Windows 8 ADK tool DISM substituted AIK Windows 7 when a plurality of tools, including PEImg, Intlcfg, Package Manager and ImageX. Following up with the image processing command relationships, a large part of the original imagex is handled, unified done by DISM, while the original version of the service, language processing, such as adding drivers and applications, or to work done by the DISM. This does not mean IMAGEX not be used, in ADK installation directory, we can still see the new version of IMAGEX.EXE, if you feel a little processing using DISM is not very smooth, you can try to use IMAGEX tool. Enter "IMAGEX.EXE /?" (In quotation marks) can be obtained IMAGEX use help. Note that, the processing system 32 is preferably run in a 32-bit system environment, the process is preferably performed in a 64-bit 64-bit system environment, which is more critical aspects DISM command, IMAGEX relatively freer.

DISM very strong, there are many uses, you can search and look.