Early 2008 MacBook not connecting to the Internet (after erase and install)

Hey everyone, I've been having some issues with my Black MacBook (early 2008) for a couple of weeks now. It's intermittant when connecting to the internet.

I recently turned my iMac into a server using Apple's Server software, but decided to delete it all as it wasn't very useful in the end and just slowed everything down. I thought I'd mention that as that's the only thing I've done to the network that could be causing an issue.

I've tried everything from deleting the System Configuration file to an erase and install from fresh. As these Macs aren't able to upgrade to Mountin Lion, I wondered if there were any updates in the release notes that changed any network configurations.

It's strange beacuse my iMac, MacBook Pro (both running Mountin Lion), iPhone, iPad, and hate to say it - PC, are all connecting absolutley fine. No problems at all. I'm a web developer so I'm quite intelligent when it comes to computers 😉 but I'm now at a complete loss after all this. I can't work out what's wrong!

On the System Preferences Network Diagnostics check, I get all these different lights:

And it doesn't matter even when they're all green - it just doesn't connect to the internet. I've been thinking about phoning my ISP but I know that they will just say, can you plug it in with an enthernet cable? And it will work, and they will say it's not their problem, the router or the computer will be at fault. And considering the other computers work fine, it points to the computer. But I've just done an erase and install.

Are there any tests I can carry out to help solve this issue? Any help would be really appreciated.