It sounds as though the partition might be corrupt. FAT partitions are, well, pretty bollocks comared to modern ones - a lot less robust. However they're easily read cross-platform, hence why its used. Did you accidentally unplug it without ejecting it?

If you don't, there may be part of the file left in the computer's cache that hasn't been written (it does it to make it look faster). XP *does* have a quick eject mode for removable media (i.e. it doesn't cache it), but I'm not sure if that's enabled by default.

When it's plugged into your computer, right click on it & reformat as a fat16 (or just fat) partition.

Hopefully that should reset things to 0.