file sharing problem

hi, first i am new to mac. i am trying to share files on my external hard drive, which is connecting to my mac, with my pc's. here is what i did:

1. go to system preference->sharing
2. click file sharing, now the status of file sharing is "on"

however, i can not see any folders under "Shared Folders" (even the public folder),,, what's more, i can not add any folder into the "Shared Folder" box.

if i click "+" under the "Shared Folder" box, and select some folder, the folder i selected will not shown in the "Shared Folder" box, and my mac will halt: i can not start any application, and even i can not force quit the "system preference". what i can do is to turn off mac by press the power button,,,,,

does anyone have the same experience?

thanks in advance!!!

Don't know if you still need help with this, but if you need to share with Windows computers, make sure "Share files and folders using SMB (Windows)" under options in the Sharing preferences pane.

It sounds like you may want to run "repair permissions" in the Disk Utility application.