Look at /etc/fstab, and you must see the option "rw" in the lines refering to your devices (flash or other partition), like this:
/dev/sda /mnt/flash auto noauto,user,owner,rw 0 0 /dev/sda1 /mnt/flash1 auto noauto,user,owner,rw 0 0
If your Windows XP partition is ntfs, you must recompile the kernel to support write on ntfs partition (CAREFUL IS EXPERIMENTAL), if not is an ntfs, try putting the rw option.