At least that's what caused it according to Windows.
Here's the Event log:
The computer has rebooted from a bugcheck. The bugcheck was: 0x00000019 (0x0000000000000020, 0xfffff8800d5d89b0, 0xfffff8800d5d91b0, 0x0000000005800d5d). A dump was saved in: C:\Windows\Minidump\Mini030709-01.dmp.
Windows sent me to:http://wer.microsoft.com/responses/R...8-506d4f28d745
Operating System:Windows Vista 64 bit
Product Name:ZoneAlarm Internet Security Suite