Our speedstream 6520 modem won't con...

Hi we have a speedstream 6520 and the older computers connect fine to it with our WEP code - however we just bought a new laptop with vista and can't find the disk to the speedstream - it says the wireless strength is fine - however the new laptop keeps dropping in and out of connection when on wireless - its vista based the old ones are windows xp - however I think I may need the software or disc - anyone got ideas - I just don't understand why other older laptops can connect immediately with just our WEP code - but this new one needs the software installed???