1. If you are on new Start Screen, type power and Windows will automatically search for the term and will show you the results.
If you are on Desktop, move your cursor to top-right corner of screen to access new Charms Bar. Now click on Search option and type power in search box.
2. Now click on Settings link given in right-sidebar of search results page and then click on "Change what the power buttons do" link given in the search results as shown in following screenshot:
3. It'll open Power Options window. You can alternatively open the same window by typing powercfg.cpl in RUN dialog box and press Enter and then click on the "Choose what the power buttons do" link given in left sidebar.
Now scroll down to bottom and you'll see "Turn on fast startup (recommended)" option given in Shutdown settings section as shown in following screenshot:
4. Disable the option by unchecking the checkbox and then click on "Save Changes" button.
PS: If the option is not shown or greyed out, scroll up and click on "Change settings that are currently unavailable" link.
5. That's it. Restart your system and your reboot or shutdown problems should get fixed in Windows 8.
NOTE: If the above solution doesn't fix your problem, you can try to execute following command in Command Prompt. Open Command Prompt as Administrator by right-click on the bottom-left corner and select the same option from the Win+X menu:
bcdedit /set disabledynamictick yes
The above mentioned command used to fix a very annoying freezing issue found in Windows 8 Release Preview build. According to Microsoft the issue was fixed in Windows 8 RTM but if you are still facing the issue, you can try the command.
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