Manage permissions for automounted windows partitions

First of all, here is my fstab:

# Pluggable devices are handled by uDev, they are not in fstab
/dev/hda3 / ext3 defaults,noatime 1 1
none /proc proc defaults 0 0
none /proc/bus/usb usbfs devmode=0666 0 0
none /dev/pts devpts mode=0622 0 0
none /sys sysfs defaults 0 0
-> /dev/hda1 /mnt/win vfat,ext3,ext2,reiserfs auto,exec 0 0
-> /dev/hda5 /mnt/data vfat,ext3,ext2,reiserfs auto,exec 0 0
# Dynamic entries below, identified by 'users' option
/dev/hda4 swap swap sw,pri=1 0 0

hda1 is my Windows partition, and hda5 is a fat32 partition that I use to share my files between Windows and Linux. I wanted these two partions to be mounted automatically at boot, and it works, but I also want to manage permissions for /mnt/win and /mnt/data. In fact, only root can read/write in these repertories, and if I try to change permissions when they are already mounted, an error message tells me that I don't have permissions to do it, even as root! Instead, if I change the permissions when the partitions aren't mounted, it works, but at the mount time, the permissions are automatically changed back.

To control the access for the different users, I created two users groups: mnt_win and mnt_data. Mnt_win must be the owner group of /mnt/win and mnt_data must be the owner group of /mnt/data. Furthermore, I want the permissions of the two folders to be rwxrwx---.

So, how the hell can I do this? Your help will be appreciated a lot since I passed almost an entire week on this one.