If you receive messages (in diverse ways) saying that your system cannot find Explorer.exe, start Regedit (possibly in Safe Mode Command Prompt Only), and go to:
HKLM/Software/Microsoft/Windows NT/Current Version/Image File Execution Options/explorer.exe
Look for a subitem named “Debugger”, and remove it. Then reboot.
This reportedly happens a lot when antispyware applications remove infections without fixing the above issue.
If it happens again after the reboot, your antivirus is causing it, by unsuccessfully attempting to remove antispyware, repeatedly. In this case, do the registry change, but don’t reboot: just run Explorer.exe from the command prompt. It will run. Then run something else to clear out the spyware.