Some NICs are picky. Try going to the settings and hardcode it for 100mbps, full duplex. Odds are that it is set for "Auto". It is having trouble auto negotiating with the hub/switch. I see this a lot.
The speed from the Xbox to the PC may be slower because you are ripping from a CDRW/DVD. The bottle neck is the read speed of the CD/DVD drive on the Xbox, not the connection speed. My speed from PC to Xbox is also double that of Xbox to PC. Usually I am ripping while I am doing it. I haven't tried to FTP from the Xbox HD to the PC HD.
I have several CDRW burns I have done that experience the painfully slow transfer speeds. I think it is caused by the drive having difficulty reading the CDRW. Maybe try burning it again. I gave up and didn't bother with it. You can also unrar the iso, and then use Xiso to extract the files on the PC, and then transfer over.