I can mount the Windows partition in my Ubuntu installation but I don't seem to be able to create or paste files in to the mounted partition. I don't know how to fix it. Any ideas?
If you mounted from Nautilus, it uses a special GNOME API (gio/gvfs). You can't access the files from command line or non-gnome-aware programs. But there is a bridge:
You should install gvfs-fuse (