log into the router from your web browser using the ip like
http://192.168.0.1
if you don't know the login google your router model # and find the login (admin, password or admin no password.
then go into wireless settings and lookup the SSID, security type and KEY.
you probably can't read the KEY so use NOTEPAD to type up a key then paste in the new key and save the router settings.
this will disconnect any computer wirelessly connected as the key is changed.
now go into the wireless setup on each computer and use the NEW KEY
even with a lame MAC this should be easy. on XP it's idiot-proof beyond words

...glen