Problems connecting to the iTunes Store often stem from issues with the Windows firewall or a firewall in a router, a privacy filter (such as McAfee Internet Privacy Service or Norton Internet Security), an antivirus/antispyware utility, or a download accelerator. Start here:
for some basic tips to start isolating the cause of the problem. What you may need to do is temporarily disable any such service and see if that fixes the problem. If so, then if it's a firewall/Internet filter that's causing the problem and you'll need to set the offending service to allow iTunes to connect to the iTunes Store.
Hope this helps.