If you can run both the WiFi and normal ethernet adaptors in your laptop at the same time (believe it or not, some lappies wont allow this) then its a fairly simply matter of bridging the ethernet to the WiFi.
For it to work, you would obviously need to keep the lappy powered on. A better solution may be one of the alternatives suggested above or perhaps something like a set of homeplugs (ethernet via your mains cabling).
If your trying to route through your lappy then I presume you have some kind of reason for doing so. Hijacking somebody else's WiFi by any chance ?
Bring up network properties in control panel. Right click your wireless connection. Then the Final tab along the top should be for advanced settings you should see Internet Connection Sharing in there. Tick the box and I think that should be it. If you are still stuck let me know and I'll fire up one of my XP VMs to check. Going to dinner now so it will be later on.