Here's what another site says about rebooting instead of powering down:
Booting and rebooting ? What to do when Windows reboots instead of shutting down
All Windows versions
On certain machines exiting Windows lands you in a "reboot loop" instead of turning off the computer. To fix this, go to HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer
and set the DWORD CleanShutdown to 1 instead of 0.
Link:
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Here's a web page with a bunch of links to articles on shutdown/reboot problems:
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