This issue occurs because the IUSR account is disabled. If you go to Computer Management > Local Users and Groups > Users, the IUSR account on the OfficeScan server is disabled. 

To fix this issue:

1. Untick the Account is disabled check box for the IUSR account.
2. Restart the IIS Admin Service and World Wide Web Publishing service.
3. Access again the OfficeScan web console. The Windows prompt will disappear.