Really only one simple method: in XP on the laptop use Windows Explorer and copy the root-level folder on the external HDD to the NL disk (assuming its capacity is larger than usage on the external HDD connected to the laptop -- otherwise you'll have to split onto multiple NL disks).
I recommend the laptop is cabled into your network (although if you have wireless "g" you'll be ok).
Because NL uses ext2 or ext3 filesystem, there really is no faster way (that's still easy). (Before someone comes back with ... boot a live linux CD ...)

Georg