Windows informs you that there are users currently logged on when you try to shut down your computer. That is normal if there are additional users logged in Windows. However, to perform an in-depth clean, try using CCleaner.
Download and install the application, then tap on Cleaner and Registry. It will delete all the temporary files and registry items that are obsolete.
Secondly, run a copy of AdwCleaner or Combofix to scan for for possible malware or trojans.