Ad-Hoc w/l connection drops out (Vista (ICS Host) -> XP)

Hello
I have a Vista Ultimate that is connected to 100mbit with wire. Then i use
ICS to give a laptop with XP sp2 internet using WLAN. I have no router so the
desktop i directly connected teh intarweb and the laptop via wireless Ad-Hoc.

The XP looses ip & gateway after everything from 3 sec to 12 hrs and fails
to renew this. I've tried to update the drivers to the desktop's 'Netgear
WPN111 USB-wlan adapter'. I've also checked power-saving of the network
adapters on both systems.

Any hints appreciated!