My thanks to all that responded to my posting about our virtual machine CentOS 6 system start-up issue. I found the alternative boot options to be the most helpful. Interrupting the boot-up process with Alt-d or Escape allowed me to see what appears to be a quite normal string of start, install and mount activity. However, this process ends with the system hanging at the point below:
Starting ipmidetectd: ipmidetectd: No nodes configured [FAILED] Starting sendmail:
It is not clear to me whether the boot-up process is hanging due to the failed starting of ipmidetectd or sendmail, but I suspect that the ipmidetectd start up failure is the actual cause. It is not clear whether any IPMI related features were ever installed.
Interrupting the boot-up process and selecting Run Level 1 results in a functioning system. Starting with other Run Levels results in the incomplete boot-up process noted previously. Using the "service network start" command yielded functional network and internet connectivity. This enabled the successful execution of yum update. The update consisted of kernel and other updates with a total download size of 274 MB.
After this update, the boot-up process still hangs at the point indicated above. Why this is happening is still a mystery and if it actually is IPMI related, why would this be appropriate or even needed in a CentOS system that is running on VirualBox. There is no IPMI related hardware accessible to the virtual CentOS system or on the Windows 7 host system. It would be good if the IPMI start-up could be disabled.