I've finally found a solution that will make the Fast Debugger (a.k.a ruby-debug-ide) work with Netbeans 6.9.1+ on Windows 7. This solution should also work for anyone using Vista, which I know from experience has similar issues.
First step: make sure the Ruby DevKit is installed. Full instructions for installing it are here on Github: https://github.com/oneclick/rubyinstaller/wiki/Development-Kit.
The DevKit basically temporarily enhances Windows and allows tricky Gems to be compiled.
Second step: Next you must install the correct debug Gems. If your Ruby version is 1.9.2+ then you will require the gems ending with a "19".
The way to make the gems install successfully is through Netbeans. Go to Tools > Ruby Gems > New Gems.
Do a search for "ruby-debug" and you shall see a list similar to this:
Install the "latest" version of all of the Gems which I have highlighted. Do not try and install "ruby-debug".
Third step: Once the Gems have installed, open up and new Command Prompt, run the command "gem list" and check that all of the Gems are listed, if not try to install them manually.
Next, the "rdebug-ide" file must be edited. Find this by browsing to (or similar):
Find the line (around 79):
Debugger::PROGSCRIPT = ARGV.shift
Replace it with:
script = ARGV.shift Debugger::PROGSCRIPT = (script =~ /script([\\/])rails/ ? Dir.pwd + $1 : '') + script puts Debugger::PROG_SCRIPT
Debugger::PROG_SCRIPT = ARGV.shift
Final step: Save the file and restart your computer. When Netbeans is reopened, click the "Debug main project" button to start the server in debug mode.
Thanks must be given to the following for help in figuring this one out: