This is the sort of thing that comes with a big red warning so:
1) Backup your data
2) Check that your data is backed up
3) Double check that your data is backed up
4) Try using ntfsresize from ntfsprogs:
http://linux-ntfs.sourceforge.net/
5) If it works you're done, if it doesn't...
6) Delete the small partition and create a new one
7) Format the new partition
8) Restore the backup you made in step 1.