Restarts instead of shutting down

My computer restarts instead of shutting down????

today when i press shutdown from the start menu, my computer instead restarts. any ideas why it is doing this and any ideas how to fix it??
Solution:- Your computer is encountering a system error when it tries to shutdown.
It would be useful it provided some kind of error message on a BSOD (blue screen of death) but there is a setting to make it reboot on system error. The link below tells you how to change this setting... Once you have done this, when you shutdown, you should gain a little more information related to the problem - a 10 character code in a message (say) like this:
STOP 0x0000007F (UNEXPECTED_KERNEL_MODE_TRAP).
Now hold the power button in, to switch off. Then restart...
Google (or yahoo) the 10 character code.
You should (eventually) find more information about the problem and usually at least one workable solution.
Good Luck!