If you have enabled WiFi connections in your router, then you should make sure that it is secure. This is because not only anyone can easily steal your bandwidth, anyone can even capture all your traffic and get all your passwords and credit card details. For a normal user it is difficult to deploy a secure network with the router since it is a complex task. However, you can do one thing, just check from time to time who is on your network, and simply block the intruders. The simplest way to do this is to go to use this small and free utility, Zamzom wireless network tool.
Download Zamzom wireless network tool from this page.
Simply start this tool and click on the Fast Scan button. It will immediately display the MAC and IP addresses of every computer that is connected to your WiFi connection. You can then find out the IP addresses of all your machines and match it with the stats displayed by Zamzom wireless network tool. The easiest way to find out your IP address is to visit this page. You can easily find out if there are any intruders using this trick.
If after a Fast Scan, your internet connection is still slower then you expect, you can use the Deep Scan. It takes more time but it is more thorough.
Once you know the IP addresses of all the intruders machines, simply block them through your router. You can Google about how to this. Use a query like, How to block IP addresses in your-router-name.
At the time of writing, Zamzom wireless network tool is only available for Windows. It works on Windows XP, Vista and Windows 7. It even works on Windows 2003/2008 Server.