Connect to the router via Ethernet cable and change the security settings. You say you have lost the password, do you mean the login password for the router?
Have you tried the default passwords? Try admin / admin, or admin / password .
What make of router is it?
You may have to reset the router if you cannot get access to the configuration page, you will loose all settings including you ISP username and password.
May sure that you know these before resetting.
Are there any other computers that are connected successfully via wireless?
Also, if you can use WPA encryption instead of WEP.
WEP can be cracked in five minutes.
Use WPA with a long random password of at least 20 characters.