After trying to reinstall the iCloud control panel, do the winsock reset maneuver, and try turning off my McAfee (on my Win 7 64-bit Dell) and my ZoneAlarms on my old XP Pro dual core 32-bit Dell, I still could NOT get past that error message. I put requested a call from Apple, and just before getting that call, I tried one more thing... I created brand new users, made no changes to the user settings, and went straight to Firefox and logged into iCloud.com, and I was able to get into my account on BOTH the Win 7 and Win XP Pro machines using the new user accounts. The original accounts still fail.
I figure that this is not a problem with the router, and may not even be related to the security software (since when I launched Firefox on the new user accounts, I did NOT deactivate the security software first)... It is a bit of a work around, but switching users to log into iCloud is relatively painless.
When Apple called me a bit later, I explained the original problem and they are aware of it and are working on it. Their experience so far is that it is usually the security software, and they recommended removing McAfee or ZoneAlarms and using Microsoft's free Security Essentials. The Apple tech figured that there was some setting in some third party software that was causing the glitch, since all of the same software is still on both machines, but the only difference is that none of it has ever launched under the new user names.
So anyways, the bottom line is that Apple thinks it is likely related to your anti-virus/anti-spyware/firewall software, and they recommend switching to the Microsoft security software... I'm not sure I agree, but there is some setting on my original logins that iCloud doesn't like, and you might want to try creating a new user on your PC dedicated to iCloud.
Good luck!!