I'm not exactly sure bout ur problem but these might be what causing it..
1) u're using WinXP with no service pack or service pack 2 with no support for WPA2. change wireless security to WPA or u can apply this update - http://www.microsoft.com/downloads/en/deta...e8-f7fc20195769

2) Your wireless router is using wireless channel 12 or above. Ur computer's wireless adapter probably unable to detect wireless router with that channel, only limited to wireless channel 1 to 11 (happened to one of my laptop before). Try check which wireless channel that ur router is using.