If you edit your etc/fstab file as root, and enter an "ro" parameter next to you NTFS partition(s) then they should mount only as read-only.

i.e. for hda5:

/dev/hda5 /media/hda5 ntfs noauto,ro,users,exec,umask=000 0 0

Make sure you make a backup of your fstab file if you change it though, in case all hell breaks loose!