Let me get this straight, you have WIRED connection between your computers and your laptop have wireless to the WAN (internet)?
IF so, can you first able to ping your wired port of your laptop from XP? If it's okay, then all you need is to turn on NAT features on Linux box. The easiest way to do this to install firewall Firestarter and let it take care of it.
Two advantages of using programs like Firestarter,
1. You get a secure firewall
2. It's GUI based so it takes care of a lot of typing and command line confusions.