It might be due to any virus or malware that has been installed on your computer, incidentally. Try to scan your computer thoroughly with the latest updated antivirus. If the problem persists, you can follow the workaround given below.
Create a new user account. Log on to Windows 8 with the newly created user account and try to shut down your computer from this account. If this works, move your required data that resides in C:\Users\<OLDUSERACCOUNTNAME>\Documents and C:\Users\<OLDUSERACCOUNTNAME>\Downloads folder to the C:\Users\<NEWUSERACCOUNTNAME>\Documents and C:\Users\<NEWUSERACCOUNTNAME>\Downloads respectively. Once done, delete your previous user account.
Let me know if this helps.