After numerous problems caused by Apple's awful iTunes upgrade I've descovered the root of the problem and a workaround.
Many people have discovered that after updating to iTunes 5 they lose net access. This seems to effect users with a D-Link DSL-G604T router, but it may effect others as well.
The cause seems to be the Bonjour Service aimed at helping people share music across a network, but Bonjour works by handing out spurius IP address willy nilly, it change my DNS server address to something silly like 184.108.40.206.
Whilst this should not happen, it does, and it's tres annoying. A workround is to disable the Bonjour Service by wandering off to Control Panel/ Administrative Tools / Services select Bonjour Service and set it to disabled. This will stop the net problem but have a side effect of preventing iTunes sharing stuff across a network, you'll have to decide which is more annoying - rebooting your router every 5 minutes or not being able to share your music across the network.
Hope this helps
This thread is now locked and can not be replied to.