Just use internet sharing in OS X. System Preferences -> Sharing -> Internet -> Click the drop down menu next to "Share your connection from:" and select ethernet -> click the box next to "Airport" in the box below.
Then click "Airport Options," configure your new wireless network, and connect to it through your Windows XP laptop. Viola.
If you want another option, you can buy a wireless router (I highly recommend Linksys routers), and set it up between your cable modem and your iMac. You can connect your iMac to the router by ethernet, and connect your Windows laptop to the router through its access point function.