Hi, You can use a USB wireless adapter Probably one which supports windows XP. Here are a few  Netgear 150MBPS  DLINK 150Mbps <ADMIN NOTE: Broken link has been removed / replaced from this post by Dell>  Netgear 300Mbps   These are just for your reference you could either purchase it online or walk up to a near by Retail store. The term is 'USB Wireless Adapter' and your computer would be wireless capable.  
I want to know if I can get wi-fi connectivity on my Dell Dimension 3000. I bought it in 2005, and I don't think wi-fi capability was built in. Thanks,          Walter 
wi-fi connectivity