Profile restarting everytime i Logon

I have an average domain running Server 2003 and XP clients and I use group policy to control my users. Recently one of my users experienced a problem of restarting every time she wants to log onto the domain.At times the machine restarts as soon as she press Alt+Ctrl+Del on the machine.I change the machine and the problem continue even if she log to any computer on the domain. I deleted the Profile and the problem still occurs. I have Symantec Anti virus Enterprise edition and it is up to date with defs.What could be the problem