Cannot access itunes store or check for updates

[theodivine]

Hello. I wasn't quite sure where to put this, but I have had a problem accessing the itunes store and checking for system updates on every network I've tried lately. itunes tells me that the host is down, and the update checker tells me that there is a network problem. I can obviously access the internet, so I don't understand why these two won't work. I took my computer to a local mac store, but they didn't tell me anything. If I have to, I'll take it back to them and pay the $60 that it will cost me, but if there is anything that anyone can tell me to help, I would greatly appreciate it. I've tried everything, but nothing is working!

Please help!

Theo