Computer restarts itself after shutdown

I have a Gateway computer which I have had about 2/1/2 years I am running Windows XP, with a Pentium 4 and 768 in ram. About a month ago when I started my computer I got the dos screen with a message saying "windows did not shut off correctly" and the message gave me several options to restart. normally, safe mode and last known good configuration or something like that. I finally got up, I called gateway and they had me boot up using my cdrom with my drivers CD and run a scan on my c drive which it said I had corrupted files and then repaired them. Since them when I go through the normal shut off procedure (start, turn off computer, turnoff). The computer shuts off but restarts itself. I have been in Bios screen and have been to system properties, advanced, start up and recovery, settings, system failure and unchecked auto restart. Nothing has worked. I even checked my fan to see if it was working and it is. One other thing I have a cable modem and if I disconnect the cable modem from the computer the computer will not restart. Any suggestions on how to get this thing to shut off correctly?