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.
Another possibility is an erroneous entry in the Hosts file. This tip from Mark Holly resolved problem for another poster (please note that I have not tried this fix so I cannot vouch for nor support the suggestion: I provide it only as a suggestion that worked for someone else):
NOTE: this will change a Windows operating system file. This could cause you problems if not done correctly.
Using Windows Explorer navigate to the following folder:
In this folder is file called 'hosts'. Open it using Wordpad.
On the poster's pc there was in this file one long line. At the end of this line was "127.0.0.1 phobos.apple.com:. phobos.apple.com is the web address for the music store. What "127.0.0.1 phobos.apple.com" is doing is telling the pc to look for the web address only on the computer rather than the internet. By remove this section of the line and saving the changes you may then be able to access the music store without any problems.
IMPORTANT: Before making the changes to the hosts file I highly recommend making a backup of it. That way you can restore the original if other problems occur.
Hope this helps.