OK, your computer does not appear to be blocked from accessing the store and iTunes can make a secure connection.
These have been the trickiest and no clear solution has emerged. There is a suspicion that the problem may not be in the PC at all. Possibly the router or ISP.
I see you are using a laptop. Is there any possibility that you could try it at a friends location, preferably one who uses a different ISP and who can access the store?
But there are a couple of things to try first.
DOes your ISP allow you to connect to the internet without a proxy server? If so go to Internet options>>Connections>>LAN settings.
Uncheck "Use proxy server..."
You should also have the top checkbox ticked - "Automatically detect settings"
If you have to use a proxy server to connect to the internet you probably need to take this problem up with your ISP - especially if you are able to connect to the store from another location.
This page may be useful to your ISP - if give the various TCP and UDP ports used by Apple:
http://support.apple.com/kb/TS1629
Always close iTunes and reopen it before checking to see if changes have worked.
The next thing to try is to go to Internet Options>>Advanced an uncheck "Check for server certificate revocation" - that seems to have worked for a few people.