how frustrating!!
when your HD died did you upgrade to Snow Leopard? the interface is a tiny bit different from Leopard - but its the same underneath.
ok so if its the router it could be MAC address blocking like i said, or perhaps IP address issues, or perhaps your router just hates your computer lol!
so please go into your router at address 192.168.0.1 (from a computer that has internet) and have a poke around. maybe choose a new wireless password (make sure you're not in the router password section), or even a new wireless method (change from WEP to WPA or vice-versa). it might even be an idea to completely remove the password to see if that works at all.
also, on the laptop that has no connection - go to System Prefs->network->airport->advanced-> and find the "preferred networks" list on the "airport" tab. if you see your network name there, delete it. this will remove all memory of the network. click OK, then "Apply" then try to reconnect to the network again.
hopefully one of these will help haha!