Just to clarify and avoid any confusions, I have only added the Wi-Fi card so that OP can use the desktop just in case he wants the desktop to wireless connect to the router. Mind you, Title of the Thread is 'xp wireless adaptor'
and also this 'what hardware will I need or method to accomplice this ???'
This is not also very clear. Now, we don't know he wants file sharing, perhaps ICS?
I also have a desktop computer hardwired to the same router.
I will need to connect wirelessly to the desktop hard drive.
I think it's best to wait for the OP to clarify things out.