ok, now this is a long shot but here's what works for me.
I was having lots of problems like you when I first tried FTPing to my Xbox, and I finally got it worknig by doing the following:
In Windows XP, go to View Network Connections (Start>My Computer>View Network Connections)
DISABLE things like "Local Area Connection 2", "Local Area Connection 3", etc.. making sure not to disable the regular Local Area Connection. Make sure you have that one set up with the right IP address and Subnet mask, and try connecting to your Xbox.
It's the only way it works for me.
On a side note, and odd thing about my computer is that I have Local Area Connection, and Local Area Connection 4, yet I only have one etherenet port coming out of my computer. I have no idea why it's like this.