For whatever reason, Apple has deemed the 40/64 bit networks to be so insecure as to warrant dropping them from support in 10.5.  You may still get them to work occasionally, but with each new update to 10.5 it gets harder and harder to connect and to stay connected to those networks.  If you are not sure about the type of security that you are using, a simple check is to look at your password, if it is 10 characters long and made up of the numbers 0-9 and the letters a-f, then you probably have a 40/64 bit hex password.  If your password is 13 or 26 characters long, then you likely have a 128 bit password and you should be fine.  It may be possible to get the Mac to connect to those networks using some basic tricks, such as putting a dollar sign in front of your password to indicate to OS X that you are using a 40/64 bit hex key, for the ASCII keys you may have to put the password in quotes – 40/64 bit ASCII keys tend to be 5 characters long.  Your best bet is to change your security to the newer WPA security or if you must use it, a WEP transitional password made up of exactly 13 characters.