-click on start>>Run>>type 'cmd' on Run.
-on the black screen, type 'ipconfig'
-take note of Default Gateway
-pull up an internet explorer and type the Default Gateway on the Address Bar
-type Username and Password (for Linksys, use 'admin' for PASSWORD)
-at the router's setup page, look for the Wireless tab
-look for the sublink Wireless Security
-under Wireless Security choose your security options (WEP for XP, WPA for vista)
-after changing the wireless security settings, reconnect wirelessly with your computer.
-NOTE:configuration on router MUST BE DONE WIRED.
hope this helps.