If your PC or router is using proxy settings, try connecting to the iTunes Store without using an Internet proxy.
On a PC, you can use the built-in diagnostics in iTunes to test your connections and generate reports.
Find a more complete list of ports used by Apple products.
Even if all necessary ports for a feature are open by your PC's security software, you may need to configure your security software to allow specific services to run.
The steps used to properly configure the security software will vary; to find the exact steps, visit the manufacturer's support website or otherwise contact them for assistance.