Not to worry. Its not so difficult to know the network security key. First of all, you are gonna log on to your router's page. (1) First open the command prompt (black window), type "ipconfig" without the quotes and hit the enter key.
(2) Find the default gateway IP address (it should be generally 192.168.1.1 or something like this.
(3) Type the same ip address to any browser's address bar and then login. Normally the default login id and password should be one of them admin/admin, admin/password, admin/blank. Or if you have changed, you know it.
(4) Now, check under wireless and you would be able to find as which encryption algorithm have been used (e.g WEP/WAP/WAP2 etc). You may feel free to change it as well and remember it.
Any more info you need, feel free to post your queries...........