We had a similar problem on our brand new Windows 8/8.1 Dell computers: after the UEFI Dell boot logo we got a black screen for about 5 minutes before we were able to log-on. The system event log listed related time-out error messages for various Windows services (not always the same ones during each reboot).
After extensive testing and troubleshooting it turned-out that Microsoft Forefront Client Security was the culprit. This Microsoft antivirus tool was automatically deployed through System Center Essentials 2010 and Forefront Protection Server, hence we had to disable this for Windows 8 clients, and revert to the built-in Windows Defender. I assume that Microsoft Forefront Client Security is not (yet) supported on Windows 8/8.1.
Peter

