Basically, you should be able to just access files and folders shared through Windows. This is for XP. In Windows 7 it's slightly different I guess (haven't used it yet). Here's a Microsoft video tutorial for Windows 7.
First off, enable simple file sharing in Windows, by going to Tools - Folder Options from any folder and enabling the following checkbox:
Then right-click the folder on Windows you want to share, click Sharing and Security, and enable the following two checkboxes:
From your mac, in the Finder, go to Go -> Connect To Server or press Cmd+K and enter the Windows computer's name. Or you might be able to see it in your Finder sidebar, then you can just click it and it will list all shared folders:
You can then just normally browse the files in this folder and also write to them.
Safety Note: I wouldn't share your whole C:/ drive as this can cause unexpected behavior. Imagine someone is on your network and just deletes important files from C:/ without you knowing it. Rather share only one folder your Mac and others can access.