this is gonna be a tough one mate!....hopefully i can make this as simple as possible,lets try a few simple steps first,you havent mentioned whether you are using xp or vista,in anycase its almost similar,im gonna consider you using xp
1> check whether your wireless on the computer is turned on,most laptops have a feature where you can switch on/off the wireless on the laptop,(manually)
2>press the Fn+F2....normally the wireless on/off for software control
3>right click wireless icon on the lower right corner where you have the time displayed,you will get an option "VIEW AVAILABLE WIRELESS CONNECTIONS"
check whether the computer is able to detect other wireless in your area
4>do you have a third party software that manages your wireless connections???, if so try disabling and allow windows to manage the wireless connection and see if that works
5>try the system restore option
6>hardwire the laptop directly from the wireless router and check and see whether you can connect to the internet,if you can most likely it has to be with the wireless settings and not a problem with the operating system