ok, click on your desktop (to select "finder" as the active program, if you get what i mean)
Hit option+k (the apple key), then ping, up comes a window wich will show all the computers, and windows workgroups connected to whatever your ethernet cable's connected to. Obviously, you can only access the folders you've got shared.
The other option is to do it from your pc - if you go into my computer, click on the tools menu, then "map network drive", then type in the following:
\\xxx.xxx.xxx.xxx\[username]
where xxx.xxx.xxx.xxx is the local IP of your mac (get the IP from network in system prefs - will probably start with 192 if its not self assigned)
[username] is the username you chose when you first turned on your mac and installed OSX. You'll be asked for your system password too to access the whole hard drive. If you login as 'quest' you can only dump stuff in the drop-box (/users/[your username]/shared)
Your mac harddisk will then appear in My Computer and you can access it like you would your pc's harddisk.
hope this helps
Oh yeah, be advised that you might have a couple of problems copying files accross to the pc if they have certain characters in their names (\ /:?*"|<>), you'll get a "file not found" error or similar. Just get rid of the offending character and you'll be fine.
(EDIT: note how much easier it is to do it from the mac!)