I have a VFAT partition that I use with Win2k that I have made available under linux by adding:
/dev/hda5 /mnt/win2k VFAT default 0 0
'ls -l' should show you that only the root user can write to that disk given those mount options. Others have suggested alternatives that may help you. If you're not really using "default" (and maybe you're not since you say you can read and write to the drive...), then you might have another issue.
However, I downloaded some files .java from the 'net, saved them to my home directory (/home/trevor) and then tried to drag and drop them from my linux partition to the aforementioned /mnt/win2k partition. KDE and/or linux didn't like that.
You should always get some error message which is more descriptive than "linux didn't like that".
One problem you may run in to, which I did, is that Windows may somehow mark the local user "home directories" as unwritable. I don't understand how, but I had a VFAT fs with Win2k where I could write to any part of the disk except "/Documents and Settings". No file or folder under there could be modified by any user under Linux, including root.
You might just need to save your files to some other folder.