MacBook Pro won't connect to preferred networks automatically

I have an Intel MacBook Pro running Snow Leopard. It refuses to connect to my home or office wireless network automatically. We have other Macs and pc laptops that don't have this issue, so the problem is almost certainly machine specific.

On startup I have to manually select the wireless network, it remembers the key and connects fine until the laptop goes to sleep or is shut down.

Things I have tried so far:

• Ran software updates
• Ensured Remember networks this computer has joined is enabled under Network Preferences > Airport
• Dragged main wifi network to top of preferred networks (also tried deleting all other wireless networks)
• Turned airport off and on
• Recreated network location and deleted ethernet, firewire, etc.
• Reset keychain
• Repaired disk permissions
• Reset network preferences (by deleting files in Library/Preferences/System Configuration)
• Tested various wireless channels on our BT Business Hub and D-Link wireless router
• Created dummy mac admin account and replicated the same fault

The problem seemed to occur shortly after connecting to a public wifi network whilst abroad. Short of an operating system reinstall I believe I have tried everything within my capability.

Would appreciate any input or similar experiences.

Many thanks!