It tries port 80, and if access is granted it uses that. On Vista and up, you're never granted access to port 80 unless you run as administrator. So as a workaround, it tries ports 50000 - 50100, which are allowed, and are "private ports" (no one application can claim them for sole use).
Even on Windows XP, if you're already running a server on port 80, it should block Construct 2 and get it to use a port in the 50000 - 50100 range. Does this happen or does it conflict with the server? If it conflicts, I'll simply turn off port 80 for the next build and have it always use 50000-50100.