Presumably you are trying to connect your son's new PC "wirelessly" to the router, in which case it will be the WEP or WPA encryption key for the wireless network that you're being prompted for.
Connect any of the computers to the router by ethernet cable, NOT WIRELESSLY, type its [the router's] IP address of 192.168.2.1 into your web browser and this will open the configuration pages. By default, there should be no Password set so, if prompted for one, just click the button to continue.
Once in the configuration pages, go into the wireless security settings, where the encryption key will have been entered. In some routers, this is shown as plain text and you can make a note of it. In others, it is "masked" with dots or asterisks. If it is "masked" you will need to enter a new key, make a note of it, save it in the router and then use it to "Connect" wirelessly with each of your computers. The current settings in the computers will now be invalid and you need to remove the existing Profiles click here and go through the procedure to "Connect" again.
For 64-bit WEP use 5 ASCII or 10 Hexadecimal characters.
For 128-bit WEP use 13 ASCII or 26 Hexadecimal characters.
For WPA use between 8 and 63 ASCII characters.
The Hexadecimal characters are "0" to "9" and "a" to "f" only, a total of 16 available characters. ASCII characters are all of the letters, numbers & symbols on the keyboard. For WEP, Hex characters should be used in preference to ASCII.