You can start up your Mac in a "Firewire target disk mode" which tells your mac not to actually boot, but instead to pretend to be a firewire hard drive.
To do this, you do the following:
- Connect up the firewire cable, as you've described.
- Switch on the Mac while holding the 'T' button.
- Your Mac's hard drive will appear as a mountable drive on the windows machine. (provided it knows how to access a firewire hard drive, that is).
For more detailed instructions, check AppleCare document 58583 (just type the number into Sherlock's search field in the AppleCare channel).