I found an article I feel may help in this situation. Named iTunes for Windows: can't access the Internet if proxy settings are incorrect, it can be found here http://support.apple.com/kb/ts1490.
Also I recommend this artice if the above does not resolve the issue:
iTunes for Windows: Network Connectivity Tests
And lastly, it is always a good idea to restart your computer and the router/modem.
All the very best,