To configure WPA for powerful encryption :

Configuring WPA wireless security on the router

First, let's log on to the router.
Open your internet browser and type http://192.168.1.1 in the address bar. This is the default IP address of the browser.
In the login box that appears type 'admin' (without quotes) in both fields. 'admin' is the default username and password.
Write down the SSID
In the Wireless > Basic Wireless Settings menu you will see the Service Set Identifier or SSID. The SSID differentiates one wireless network from another. It is the 'network name'. Write it down, or copy paste it to a text file. You will need it later. You can first set your own SSID if you like.
In the Linksys WRT54G router you will find the wireless security options under Wireless > Wireless Security. This page allows you to set wireless encryption options.

Set the WPA alghoritm to TKIP.
Type a key in the WPA Shared Key field with a length between 8 and 32 characters. Remember that a longer key is more secure. Besides that, a key that is made up of random characters is harder to crack than a key that contains real words, because real words can be guessed far easier.
Write down the key or copy-paste to a text file, because you will need it later to connect to the network.
Click Save Settings
Your computer will now be disconnected from the router. You may see a 'page not found' warning or some other error message in your webbrowser. Your router now demands that computers that connect to it use WPA encryption and the correct WPA key. In other words: you have succesfully secured your network. Now let's configure your computer to reconnect to the router using the right WPA key.
Configuring WPA security in Windows XP SP2
I will show you how to configure WPA using Windows' built-in wireless network options. Most wireless network card manufacturers create have developed their own configuration software that you can use to manage wireless connections and security. You can use whichever software you like best. The settings remain the same. Please note that you will need a computer with a properly installed wireless network card to continue.

1.In Windows XP go to Start > Settings > Network Connections.
2.Doubleclick the wireless network connection. The wireless network connections window will appear. This window shows you the wireless networks that are within range.
3.If all is right than you should see your wireless network in the list, since your router is advertising your network's SSID to the outside world. In this example I used the word 'kazam' as the SSID.
4.Click change advanced settings in the bottom-left corner of the window. A new window will appear. Select the wireless networks tab.
5.Select your network's SSID from the list and click the properties button. This will bring up the window that allows you to fill out the network key.
6.Select WPA-PSK from the Network Authentication dropdown and select TKIP from the Data encryption menu.
7.Type or rather paste your network key in both the network key fields at the bottom.
8.Click OK.

Your computer should now reconnect to the router using WPA wireless security and the proper network key. You can consider your network well secured now.