Is there a Windows equivalent to the unix hostid?
Not really, at least not exactly. HostID was usually a way to lock a piece of software to a node. Even in the later days of Solaris you could easily change the HostID, making it less locked to hardware.
You could use the network MAC, which should be globally unique. But these are easily cloned
You can also try the CPUID
There have been similar questions answered on stackoverflow
The registry key