There is library out there, unfortunately GPL licensed, that attempts to fully implement posix including fork(): http://midipix.org/. One of these days I'd like to have a go at porting postgres to it.
... and here I thought you'd be keen to instead remove all use of globals and 'static' locals to allow thread-safe state tracking, remove use of shmem, introduce threaded backends, replace use of signals, and release ThreadedPostgres.
Sounds fun, right? :p
More seriously, shouldn't Microsoft's new (or at least re-blessed and re-released with a new paint job) Linux/POSIX support offer us some options here? I suspect not - they're probably restricted to ELF binaries that won't be able to link to native Windows DLLs to get support for things like SSPI auth, native Windows SSL API use, etc. But it's worth keeping the possibility in mind.
Frankly, a library that implements fork() might cause exciting explosions when using native Windows services, too.