how i did it is go to the windows drive that you want accessible and right click on it (i did the c drive) and there is an option called sharing. click on that sharing option, and you get a dialog box which is pretty much self explanitory.
the box should have a share option (full access, partial access etc.) and then boxes for you to set the username and password. configure it and save the settings and then go into osx and connect.
to connect from osx all you need to do is press command-k and wait for rendezvous to find the machine you are connecting to, and then select the drive you just enabled sharing on, select it and enter your username and password and the windows drive should show up on your iMac.
i never could figure out how to use windows to connect to the mac, but i can do anything to the pc i want from the mac.