Win XP Shutdown Problems!

Ever since I let windows install updates and security patches/fixes when I log off or just try to shut down the system it will only reboot. I have also noticed that after extended run times ( leave system on 24/7 ) I some time find it rebooted or back at log in page. an example is I just switched from the spam laiden Earthlink to SBC DSL they said to run it continuesly for 10 days, okay no problem except that after not being on the workstation for 18 hours I found it back at the log-in screen any Idea’s gang??