"There was an error joining the selected Airport network"... Help!


Hi all,
I am a new MAC user and still does not know much about it. I have just purchased a powerbook over the internet a couple of days back and is pretty sure that it is just as good as my friend (another mac user) claims.

ISSUE: I have been trying to connect to my linksys wirelessly; however, all efforts have been unsuccessful. The only way for me to connect is to change the setting for my linksys wireless router to 128bit. But since that my other family members are still connecting with PCs, I must figure out how to configure my MAC in order to connect to the 64bit. Whenever I try to connect, the message "There was an error joining the selected Airport network" pops up... what gives??? How can I properly configure this? Please help.

Thanks all!