You may want to consider performing the following to be sure no software is conflicting with iTunes:
1.) Disable all anti-virus software you may have running
2.) Disable all firewall software you may have running
3.) Disable all web acceleration software you may have running
4.) If you are connected to the internet via an internet router, please disconnect it and connect directly via your broadband modem
This article may help you out as well:
http://www.info.apple.com/kbnum/n300870
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