I found1 that latest releases of Hoe, ParseTree, RubyInline and even RC2 of Merb depend on a updated version of Rake.
The problem is that version 0.8.3 is buggy on Windows from both user and developer point of view.
Until those patches are applied, you can install a patched version using RubyInstaller gem server (gems.rubyinstaller.org)
gem install rake --source http://gems.rubyinstaller.org
You should end with version 0.8.3.1 which will be enough for tools like DataMapper, Hoe and others work properly again.
Hope this helps.
Next: DataMapper working on Windows, wait for a bit.
1 consider this a rant, but I do my part: I show you my code.