You need to be on the same subnet as the machines that you are trying to get to, with the right subnet mask (choose your IP address / mask manually, and set them to be the same as those on the PC that you are unplugging - warning: if you do this, do not have both machines on the network at the same time)

from the go menu, choose connect to server.
type:

smb://<workgroup>;<server>/<share>

where workgroup is the workgroup name
server is the server name or IP address
share is the sharename of the share

that you wish to connect to.

Jag will come back with a dialogue box asking for workgroup (which will be already filled in), username (which will be filled in with your mac username, which is not necessarily that same as your PC username) and password (fill in the one that matches the PC username!). Fill these in and hit return, and the share will show up on your desktop.