Which way round are you trying to do it?
That is, are you trying to access your mac from the pc, or are you trying to access the pc from your mac?
If you are trying to access your mac from the pc, make sure you have Windows (SMB) sharing turned on in System Preferences->Sharing
If you are trying to access the PC from your mac, make sure you are both in the same Windows workgroup. In Leopard, you can assign the mac to a windows workgroup by going to System Prefs->Network->Advanced->WINS