If you only have Windows computers on your network, perhaps you should be better off having your Airport external drive set to FAT32. What you lose by going FAT32 over HFS+ is:
a) the ability of storing a file bigger than 4gb, i.e. some big 720p/1080p Matroska .mkv files or most dvd .iso’s;
b) sharing via AFP, your Airport will only share the FAT32 disk via SMB (arguably not a disadvantage at all);
c) some Mac functionality like saving Time Machine backups (again moot if you don’t have any Macs).
You gain however way more compatibility, so you can just unplug the harddrive from the Airport router and plug it basically anywhere with an USB port (Mac, Windows, Linux, HDTV, you name it) for playing or managing the data on it. The option off the table is NTFS since the Airport Extreme doesn’t support it.
Here I have 3 Macs, no Windows machine at all (despite a VMware install on one of the Macs) and still I have a couple of my externals formatted as FAT32 for that reason.