Feisty will not shutdown, reboots instead

[Carl Drud]

I am new to Linux/Ubuntu and everything so far has been running
smoothly. 

However in the last couple of weeks I have experienced that sometimes,
whenever I want to shutdown, it reboots. At first I thought I just hit 
the reboot button.

Yesterday I tried from a console:

$ shutdown -P now

It still rebooted.

Any ideas?

Thank you in advance.

-- 
Carl