I have no direct experience with fork & exec on win32 but I would advice caution with both (especially fork) since IIRC they're both emulated on windows and probably not as well supported by the OS as on unix.

As a side note, if I read your post correctly, you're spawning a single thread from a CGI and I assume you want to have it keep on running after the CGI exits. Is there any reason then to start it from the CGI at all? You could probably avoid a lot of potential problems by starting the UDP server as a completely seperate process (say, as a windows service)