IMac sometimes won't switch off

Question

I trust this is the Yosemite discussion... In general, my iMac, vintage late 2007 with an SSD installed has responded well to the installation of Yosemite. The only problem is that, at times, when I want to shut down the computer, it closes some programs (Safari, Mail, Firefox for example) but the shut-down process ceases. The upper menu disappears but not the Dock. In fact, if I go into the Dock and start up a program, I can regain access to the command bar at the top of the screen. Sometimes Finder is also not available and, occasionally, I have no choice but to use the on-off switch to convince the computer to shut down. It has always restarted when switched back on and after a few seconds of configuring.
This iMac has Parallels with a Windows partition, and I don't know if there could be some conflict here - under Mavericks, there has been no problem, but now, if iMac misbehaves as described above, the desktop pictures are not the ones I have for the Mac partition, but those I have in the Windows partition, even though I have not been working in Windows, and have had Boot Camp disconnected.
Is there any indication that Parallels and Win have in some way conflicted with Yosemite, or what the culprit might be?

Answer 1

"CleanMyMac" is a scam and a common cause of instability and poor performance. The developer's instructions don't completely remove it. Follow those instructions, then do as below.
Back up all data before proceeding.
Triple-click anywhere in the line below on this page to select it:
/Library/LaunchDaemons/com.macpaw.CleanMyMac2.Agent.plist
Right-click or control-click the highlighted line and select
          Services ▹ Reveal in Finder (or just Reveal)
from the contextual menu.* A folder may open with an item selected. Move the selected item to the Trash. You may be prompted for your administrator login password. Repeat with this line:
/Library/PrivilegedHelperTools/com.macpaw.CleanMyMac2.Agent
Restart the computer and empty the Trash. Depending on the version of the product that was installed and how it was removed, you may also have to remove these items in the same way:
~/Library/LaunchAgents/com.macpaw.CleanMyMac.helperTool.plist ~/Library/LaunchAgents/com.macpaw.CleanMyMac.volumeWatcher.plist
Never again install "CleanMyMac" or anything like it.
*If you don't see the contextual menu item, copy the selected text to the Clipboard by pressing the key combination command-C. In the Finder, select
          Go ▹ Go to Folder...
from the menu bar and paste into the box that opens by pressing command-V. You won't see what you pasted because a line break is included. Press return.