windows PID = 0 valid?
Open task manager - switch to the processes tab (maybe click "View", "Show columns" and enable the PID column).
PID 0 is the System Idle Process. Since that process isn't really a process and never exits, I suspect that it is always the case.
No documentation available to me, but I'm pretty certain this is always going to be the case.
It's not stated anywhere but it comes down to how the kernel handle table works. This same object is used for both process handles and process/thread IDs. It happens that handle values all start at 0x4, and PsInitialSystemProcess is the first process to be created, so it gets a PID of 4. Idle process isn't actually a process and you can't open it. It probably doesn't have a PID for most intents and purposes but most tools consider it to be 0.