MBP, AOL ADSL, XP, and LinkSys

I have a MacBook Pro, and a Windows XP Desktop. I connect to the internet through AOL ADSL (UK), which is wired to the XP via a BT Voyager 100 external modem. The phone cable goes into the modem, and another cable runs from the modem into a USB port on the XP.

The XP has a Linksys wireless card installed, which does function. The MBP, naturally, has all of the stock wireless capability.

How can I network these computers wirelessly, in ad-hoc mode (thus without the use of some sort of router), so that they can share files and internet (keeping in mind that the XP is wired, and I would like a wireless network)?

Thank you : )

Matt