If you can safe boot to a command prompt, use the registry editor to disable automatic reboot.
System Key: [HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\CrashCon trol]
Value Name: AutoReboot
Data Type: REG_DWORD (DWORD Value)
Value Data: (0 = disabled, 1 = enabled)
Otherwise do a repair install from DVD.
If that works, disable automatic restart by unchecking the box for it in the computer properties, startup and recovery dialog or editing the registry.
Once you get back into your system check the system event log to get the BSOD stop code.
The stop code will help you figure out exactly what happened in case of a repeat.
Do full virus and malware scans of your system.