Windows 7 does not support WEP wireless encryption.
(And possibly Vista does not allow, too.)

You can create connection manually, but it is probably easier to change router settings to use WPA2.

Connect PC and router by lan cable.
Usually router IP address is:, or
Default username/pass is: admin / admin.
Go to Network Wireless security settings.

You can try WPA2.

(Note (advanced): If you want to use AP in repeater mode, some AP's allow only wep encryption.)