This error message could be caused by several problems. I would first suggest that you open the registry to:
HKEY_LOCAL_MACHINE>SOFTWARE>Microsoft>Windows NT>CurrentVersion>Winlogon. Go to the right hand side and look under the 'Name' column for the word "Shell." Now, look down the 'Data' column for the corresponding string entry which should be "Explorer.exe." There should be nothing else following this name. If there is, it could indicate the presence of a virus which might have been deleted by your AV program(or a program that you removed) thus leaving an orphan startup value. Only the name "Explorer.exe" should be there.
If there is only explorer.exe there then hit the 'Start' button, then click on 'Run' and type in "eventvwr.msc /s"(without the quote marks). When the Event Viewer window appears, first click on "Application." Look for any red icons which would indicate an error; double click each red icon and note the error and how many times that particular error occurs and the time of day. Next, do the same thing with "System." Now click on "Security" and look for any "Failure Audit"s; double click each "Failure" audit and note in particular, any "Unsuccessful" logon attempts by system services or unfamiliar work groups/stations, and the date, time, and number of repetitions. Try to remember when you first saw your error message and what you did on your computer just prior to that. Look for a correlating time in the Event viewer errors. You should be able to come up with possibly a hardware(printer, camera, etc.) installation which is missing a needed file. You may need to reinstall the software needed and then do a uninstall.