same router I have.
It's pretty easy.
1. Open IE.
2. Type 192.168.1.1 in the Address bar
3. In the dialog box, type "admin" in the password section. Leave username section blank.
4. Under the "Wireless" tab, change the SSID to something other than "Linksys". Also, disable SSID broadcasting.
5. Look for a "Wireless Security" heading under the "Wireless" tab. Mine may be different than yours depending on if you have the latest firmware version or not...which you should upgrade anyway. It's really easy and can be found at Linksys's website.
6. Enable Wireless Security and choose 128bit WEP.
7. Choose a passphrase and click "Generate" A key should appear in the "WEP key" box. Leave this IE window open.
8. In your System Tray, find the wireless connection icon (Probably looks like a little monitor with some radio wave shooting out of it. Right click on it and click "View available Wireless networks" (or something to that effect).
9. In the box that comes up, it should list one connection with the name "linksys" and should have an option to add a connection. Click to add a connection.
10. Fill in the boxed for SSID and the WEP key (which you should be able to copy and paste)to match the ones on your router's screen.
11. In the IE window, click the button to "Save settings" for the router.
12. In the Wireless connection window, click Ok to add the wireless connection.

You should be ok now. And the prick won't be able to leech off you unless he finds out your WEP key and SSID.