Ah, I don't think that's a windows problem so much as a Ruby version problem. The 'official' version of webby is not ready for Ruby 1.9.
Robert Wahler has done some work on making a 1.9 compatible version of webby. You'll need both of these:
http://github.com/robertwahler/webby
http://github.com/robertwahler/directory_watcher
Alternatively, have a look at nanoc3 which shares many of the same ideas as webby.
aimee