It sounds like you know what you're doing since you have other PC's set up fine.
I'd run through your internet connection setup again, just to be sure it's set it up right.
Also check your firewall settings, and check the windows firewall settings as well.
Download Spybot Search and Destroy, install it, update it and then check for problems and remove all it finds. It's a bit better than Lavasoft
Direct link to the installer
Also reboot your PC
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