format firewire drive for Windows XP & OS X



I am trying to format a LaCie firewire pocketdrive for use on my PC (Windows XP) and my powerbook G4 (OS X) so I can use it to transfer files.
I have already tried formatting the drive as MS-DOS on my mac (this resulting in the drive not being visible on the PC) and formatting it using the Window XP disk management tool on th PC (this resulting in the drive needing to be reformatted on the mac).
I am doubly confused as the option to format the drive as MS-DOS (whic is compatible with both operating systems?) is "greyed" out in Windows XP and therefore cannot be selected.

Any advice would be great,

Thanks for your time,