For a Desktop with Windows XP, there was likely no wireless card inbuilt. Some later Windows XP systems had these as optional. You should state your model of Dell. A USB dongle should work but check that it has a Windows XP driver on the vendors website before buying. Windows XP has reached its End of Life and is no longer supported by Dell or more importantly Microsoft. Be careful with continual use of Windows XP especially if connecting to the internet. You can install the unofficial service pack 4 before connecting to the internet which will help slightly: [URL] 
I'm trying to set up a wireless connection for my desktop pc with Windows XP. I don't see any options for a wireless connection. Would a USB adapter do the trick? Looks like the easiest option. 
Question about USB wireless adapter for Windows XP