i don't have anything running 10.3 around me, but i know in 10.4, when using WEP, you need to go into network preference and instead of choosing 'WEP Password', you need to select either 'WEP ASCii" or 'WEP hex' depending on your password for the setup to connect. the difference between the two, ASCii is letters and numbers and characters like -/?, and Hex is numbers and a-f only. the only other problem i can think of is some routers use a 'pass phrase' which is then transformed into a hex password, so people think that is what should be typed in. a wep password will be either 5 or 13 characters long. count up what you know to be the correct password, and if it's different, that might be it.
a great idea at least to test the hardware. you need to log into the router and turn off the WEP password. that creates an open (unprotected) network. that would at least let you try and connect to see if the airport card is working if the steps above don't help. make sure to go put the password back after you're done testing...🙂