What version of the NT filesystem module are you using. You can easily check for that by issuing
cat /var/log/messages | grep NTFS
That should return back the version info and check with the distro's repository for a more updated version. You can check here to find out more about the driver. The other thing worth checking out is your file system table file (/etc/fstab) to see if the permissions is set for writing to it. Speaking of which, are you logged in as a normal user? Have you tried this with superuser access (root)?
As an example of such command to mount the NTFS partition here under the /mnt/Windows with readonly (-r) and umask (to control permissions), the filesystem is specified using the -t switch.
mount /dev/hda1 /mnt/Windows -t ntfs -r -o umask=0222
Here's an example of such an entry within the /etc/fstab to automatically mount the NTFS partition
/dev/hda1 /mnt/Windows ntfs ro,umask=0222
Now, if you have this similar layout, then your ntfs partition is mounted as readonly.