If when you log on to a computer that is running Windows 7 or Windows Server 2008 R2 and the logon process freezes at the Welcome screen with the blue swirl continuing indefinitely, then this article may interest you.
On hang, the computer stops responding to any network-related commands. To recover from this issue, you must forcibly restart the computer
This issue occurs because of a race condition between the Windows Event Log service and the Event Tracing for Windows (ETW) tracing functions which causes a deadlock situation.
This deadlock situation then causes the Winlogon.exe process to become unresponsive.
To resolve this issue, visit Microsoft KB2578159, download and run the available hotfix.