Okay, your pc is not wifi enabled - I expected this when you said it is a XP desktop. They usually aren't.
All is not lost - you can buy a wifi adapter for your pc very cheaply, but bear in mind that if your router is close to your desktop your connection will be much more secure and faster than using wireless.
Lots of places sell them - pc world and independent pc shops, or online such as eBay or Amazon.
This is the sort of thing:
http://compnetworking...t/findwifiadapter.htm