If you install ntfs-3g http://www.ntfs-3g.org/ you need kernel 2.6.20 or higher for stable support.
Having said that knoppix 5.1.1 uses a 2.6.19 and I'm one of those people thats never lost data or had a problem
writing to an NTFS filesystem.

In the past I've lost plenty of data on windows systems using ntfs and fat filesystems, which is one of the main
reasons why I switched to linux.

If you do create, delete, or move files on an NTFS filesystem from the live CD, make sure you wait for the commands
to complete, if youre running from a live cd configure the network and run apt-get install ntfsprogs then you also have access
to ntfsfix and ntfsundelete two very important tools.

Finally, as Harry says it is wise to backup important data should a problem occur. The knoppix 5.3.1 DVD, I belive has
a 2.6.24 kernel so the ealier stability problems should not be a concern, however if the data is very valuable its still prudent
to back it up, windows is quite capable of destroying the data without any help.