It's most likely that your wireless adapter is switched off. If you're using a laptop, can you see a tiny switch on the front or side or is there a wireless icon ((0)) on one of the Function keys? If the latter, try pressing that key and the Fn key to the left of the spacebar at the same time.
In XP's Control panel, open Network Connections, if wireless doesn't show go into Control Panel then System>Hardware>Device Manager>Network Adapters to see if it appears there. Does it have a yellow mark beside it and in Properties, is it Enabled?