Everytime I try to logoff as a user or turnoff computer using the start menu, I receive a fatal system error.  At first, my computer begins logoff as usual by stating "logging off" and "saving user setting", then all of sudden the screen turns blue with the folowing message:  STOP: c00021a {Fatal System Error}  The windows logon process system terminated unexpectedly with a status of 0xc0000005 (0x00000000 0x00000000).  The system has been shut down.   1.What is this error?  2.How can I correct it?   3.What caused the error?