Vista + ICS + XBox360 = :-(

I just can't get ICS to work under Vista. I remember I had a lot of trouble
with XP as well but can't figure out how I fixed it then.

I have a built in (Motherboard) 3Com network card and a PCI Realtek card.
Both cards work fine, and I can (evidently) access the internet. I can also
see the XBox 360 show up in the devices thingy, but when I try to connect
using the XBox it won't get an ip adress. I've tried enabling the discovery
thing, setting the second network to private, enabling internet connection
sharing on the first network, gone into the advanced system properties and
done the Network ID thing.

It just won't work. It doesn't find the ip. In Windows XP it does work.