When Windows encouners a problem, it will restart by default. If the system encounters an error durring shutdown, it will revert to the default behavior of restart. I suspect this is the case with your machine. Please take note of any errors and post them here. For now, there is a work-around:
Right-click My Computer
On the Advanced tab, click Settings under Startup and Recovery.
Under System Failure, if the Automatically restart check box is checked, uncheck it.
Make sure the box is checked to write a log file.
I expect that this will not resolve the issue, but it will log an error and shut down, hang (just stop), or bluescreen. Let us know what does happen, and post the results. Then we will look at the next step.
Microsoft says that one of these could be the problem:
-Mismatched system files have been installed.
-A Service Pack installation has failed.
-A backup program that is used to restore a hard disk did not correctly restore files that may have been in use.
-An incompatible third-party program has been installed.
So we would first ask ourselves:
-Did we manually install system files?
-Has Service Pack installation has failed?
-Have you restored any files or used system restore?
It might malware since it is identified as "serial" rather than "unknown device," but waiting for reply to be sure I do not want to have shaludas delete a driver that just needs updated. I guess a generic driver for a serial port would be better than one that hangs... and it is not likely to ever be used... but I like to play it safe.
Right-click on that driver and select uninstall. You may be warned about removing a device from your system. Allow it to continue. Once that is done, I think you'll find that your system shuts down properly.