You'll have to set up the Windows XP machine as your internet sharing proxy, which you can do via the HELP function in Windows XP. It's okay to do that, but realize your laptop wouldn't work unless your Windows XP machine was on and running.

I have a better solution for you though.

Go buy a NetGear or LinkSys wireless (and wired) Router/Switch. It will do all the "proxying" you need, and provide both wireless and wired connectivity.

You can also tell it to only allow certain MAC Addresses to connect, or password protect it; that way people can't connect to your network and steal your bandwidth.

The routers cost ~$100.00

With this setup, your network would look something like this.

Cable Modem/DSL -> NetGear Router -> Transmits to Wireless Card on your Laptop --> (simultaniously) Transmits to your wired connections via the RJ45 plugs in the router.

Your computers would work like this.

Laptop -> Wireless Card -> Netgear Router -> Cable Modem -> Internet -- and the router would take care of routing it all back to the proper machine.