This is what did it for me:
Posted by firemanli on October 25, 2011.
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1) Does rebooting the computer resolve the issue? If yes, stop here. Issue SOLVED by Apple Tech.
Here was the steps that Apple gave me to solve this issue.
2) Check to see if any of the following software is installed: CovenantEyes, PCTools Spyware Doctor, or Browser Defender. If yes, remove this software and reboot the computer; if the issue is resolved, stop here.
I did not have any of the above programs installed, but I did uninstall Spy Bot Search and Destroy. After the reboot this didn't solve the problem.
3) Open a command prompt as an administrator and enter: netsh winsock show catalog >C:\before.txt
4) Then enter: netsh winsock reset
5) Reboot the computer. Is the issue resolved?
The issue was solved after the reboot. So I am not sure if it was just a winsock reset, or a combination of the reset and removal of Spy Bot.
6) If steps 4 and 5 resolved the issue, please open the command prompt again and enter: netsh winsock show catalog >C:\after.txt
7) If the issue was not resolved by the steps above, please enter this command: netsh int ip reset c:\resetlog.txt
8) Reboot the computer. Is the issue resolved?
9) Please also inquire as to what AntiVirus, Firewall, Other security software the users knows is installed and report that here as well
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Points 3, 4 and 5 did the trick. Make sure you use the cmd command as described by ballbergen above.
This solved iCloud and iTunes sign-in on my PC (Windows 7 32 b) and my wife's laptop (Windows & 64 b). The problem disppeared entirely.
Good luck.