Help needed to set up ad hoc wireless network to share dial up connection

I am trying to set up an ad hoc wireless network to share a dial up internet
connection but am having trouble getting the internet sharing happening. I thought this would be a fairly painless thing to do - obviously not!

I am using two computers:

1 - a desktop with a dial up modem and a wireless network card running on
Windows XP Home that connects to the internet via a dialup modem.

2 - a notebook with a wireless network card running Windows Vista Business.

I want the laptop to be able to connect wirelessly to the desktop and share
the dial up internet connection (once that is working will then want to share
printers).

Both computers have wireless network cards.

I have managed to set up the wireless network and it seems to be working OK but the internet connection sharing isn't working.

The desktop still connects to the internet and works properly.

The laptop sees the desk top system but it shows that the internet is not connected to the desktop (even though it is).

I have tried a number of different IP config and DNS settings on both network cards but seem to be chasing my tail.

Can anyone offer me some assistance to get this up and running?