I have tried to delete the Userclass.dat file precisely as it is annoying to get deleted throwing pop up as : the userclass.dat is used by SYSTEM.
Here are the troubleshooting that I did so far:
1. Tried to delete the user from the User Profiles in System Properties but cannot fine that profile precisely as there are so many Unknown account.
2.deleted the profile from Profile List in registry and rebooted the system.
3. tried to disable inheritance to the .dat file and removed the SYSTEM from the Security.
Let me know what needs to be done next.