Unable to sign-in into iTunes store

Unable to sign-in into iTunes store for more than a month . The message I get is following :

The Apple ID you entered couldn`t be found or your password was incorrect.

Everything else looks OK. I`m just unable to view my wish list and purchase music or do anything else that requires to sign-in.

Possible solutions that I tried:

1) I reset my Apple ID several times. No change. Then I contacted Apple Support via e-mail and advice I got was to reset my Apple ID once more. Done. Still no change.

But I`m still able to log-on into Apple Store and My Apple ID. I`m also able to sign-in into iTunes store from my work computer, just not home computer.

2) Tried some of troubleshooting steps:

http://support.apple.com/kb/TS1368

No change.

3) Then about three weeks ago I uninstalled iTunes. Run several antivirus programs & diagnostics for my PC as I thought there could be a problem. No.

Decided to wait for the next iTunes update.

4) Few days ago I downloaded the latest iTunes version (12). Installed it. Tried to sign-in & Still got the same message.

5) Decided to create a new Apple ID. Was successful in sign-in. Added few songs in my new wishlist. Everything was good for about 30 minutes when I tried to purchase one of the songs. I was required to enter my Apple ID & password again. And got the same old message.

6) I`ve reset the password for My Apple ID for both accounts again. Still no change.

I`m out of ideas

OS is Windows 7 and regularly updated.