|Are you certain it is WEP and not WPA? |
If the encryption standard in use is WPA and your laptop cannot handle that level there isn't a whole lot you can do about it.
The modem came with a user manual? Does It tell you anything about the wireless security configuration? Since you have not told us the model number of the modem we're guessing, but in the only manual I found online it indicated that modem employs WPA-PSK security.
Goin' Fishin' (Some day)