From one of your connected PCs in internet explorer browse to the admin pages of your router, usually
http://192.168.1.1 You may be asked for a user name, often "admin", and a password, hopefully you know it
If you look at the wireless options, you should be able to see the security type that has been set from a pull down list - WEP, WPA etc. You should also see the settings for a password. If security hasn't been set you do want this to stop others leeching from your broadband, but be careful that you don't disconnect your current wireless PCs.
Now on your laptop, right click the wireless network icon on the bottom right of the page. The pop up menu should allow you to set connection options, including security type and password to match the router's settings.