Slow Logon

I have several users on each of my 20+ machines. Some user profiles will logon immediately while others on the same machine will hang on “Applying settings” for 5 minutes. Some of these I have created new users and copied the files from the old ones to fix it but every once in a while a profile will start logging on very slowly for no reason. I check for spyware religiously but I find nothing. Any ideas???