When you log into the Mac from a PC, you have to log in as a user with an account on the Mac.
Then all the drive will be available to you. They do not appear on the desktop but as a share when you first attempt a logon.
When I was doing this, I mapped a drive on the XP machine to be an external drive that was attached to the Mac.
I assume you have File Sharing turned on
I have a Mac G3 running Tiger with two external firewire drives daisy-chained and formatted for FAT32 on my home network with a desktop PC running XP with an external USB drive and a notebook running Vista. Sharing the XP external drive was a piece of cake.......just right-click on the drive, drop down box to "Sharing" and click it then turn on sharing, and I have full read/write capabilites from any machine. However, on the Mac it's not happening. I can log onto the Mac from any other machine on the network as Admin, but only the desktop shows up for sharing. Even though both of the Mac's external drive icons are on the Mac desktop, they don't appear on the desktop when logged on at one of the other machines on the network. I have turned on winoows sharing on the Mac. Is it even possible to share drives on a Mac?