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