How to connect computer to WI-FI?

I have a 2004 Windows XP, when i go to Google it says " Internet Explorer can't display the webpage ", it says i'm connected to the Internet, but i need to reconnect, with i did like 5 times… Under most likely causes, i tryed all that, i'm using i Wi-Fi box for the internet, i know it works because i have a iPad and a iPhone whitch use the Internet and it works Perfect, Please Help Me.

Added (1). Its a PC, Desktop, Computer… I'm sure Wi-Fi works for it because it used to work Great, then it started doing this…

Added (2). Yes all the lights on the Wi-Fi boz are Green and working, its connected for sure…

Is it laptop? Many people don't know that PC can't use Wi-Fi.

Desktops don't have wireless adapters, your going to have to get a LAN cable and connect it to an open port on your router.

This is the cable: http://www.newegg.com/...6812576107

Is the Wi-Fi box connected to the pc via USB/Ethernet Cable? If not, the Wi-Fi can't connect to the pc because Wi-Fi isn't built in to the pc.