Check if your wireless connection is present in Network Connections.
For Vista go to : Start->Control Panel->Network and Sharing Center->Manage network connections.
For XP go to : Start->Control Panel->Network Connections.
If you don't find it there, check if you can see the wireless adaptor in the Device Manager under Network Adaptors. The Device Manager can be accessed from Control Panel. If the wireless adapter is greyed out just right click it and click enable. If you can't see the wireless adapter then try the option "Scan for hardware changes" under the "Action" menu.
If it still doesn't detect your wireless adapter then it's a hardware problem and you need to get the physical adapter checked. It could have got damaged due to overheating or some other issue.