I spent days on this problem to pinpoint the culprit, which is Norton Security.
Can anyboby suggest how to circumvent this?
On Windows-XP with latest (well, it is latest anyway) update, I installed Norton Security trial version which was downloaded in May. Initiated the update of 73.11MB of 15 items, and it completed successfully. Windows shut down as normal at this point. Then, I rebooted and repeated the update. The size was 103.39MB for 5 update items. I shut-down the PC as normal. Reboot the PC again, wait some minutes without any operation, then initiated the shut-down. Windows comes to the point showing "Windows is shutting down" and nothing happen after it, forever.
IBM Thinkpad X60, WIndows XP professional SP3, Norton Security 188.8.131.52
I repeated HDD format, XP installation, application installtion, driver installation, several times. As the last time trial, I installed software/driver one by one, testing power on/off for each. Norton was the culprit. Uninstallation of Norton cures problem, and reinstall caurses the problem.
There are no further update now (9:00am GMT 6/28). WIndows 7 and 8.1 works OK. I need XP for maintenance of old programs that I wrote.