I have a Vista PC connected to the internet via a 3G network card that does not work with WLAN routers. Now I'd like to share that internet connection with my (XP) laptop, an iPod Touch ...

If by "ethernet is unchecked in network control panel" you mean it's unchecked in the Network Port Configuration list, go ahead and make sure it's checked. It won't show up in the Sharing pref pane as ...