Windows 7 - NetBIOS error 53 when accessing shared hard drive

From Windows 7, when I try to access my external HD plugged into my Airport Extreme base, using Airport Agent, I get "Error 53 Make sure that NetBIOS is activated"
I activated NetBIOS manually and tried a few tweaks but nothing changed
From my iMac everything is perfect: I formatted the disk to HFS+ (journaled) and did a backup with Time Machine
The iMac is connect over wi-fi while the PC is connected with an ethernet cable
I use ZoneAlarm firewall and deactivating it didn't help

Just a few minutes ago when I re-plugged the external HD into the Airport base, Windows mounted the volume for the first time and it appeared as a network drive; but when I tried to access it didn't let me and the message "No disk present in drive Z:" appeared
From My Computer the disk was then labelled as formatted FAT32 that was the previous file system it was in

I really need help to solve this problem seen that I've got two PCs with Windows 7 and my iMac connected to the Airport Extreme but only the Mac can access the disk

Anyone?