I recently got my PC Operating System upgraded from Windows Vista to Windows 7 and to my surprise the PC was no long a registered PC in my Ovi Music Unlimited account. Ovi Music allows you to change your registered phone or PC once every 90 days. Thus, in my case, I had hard reset my phone last month and already used this option to re-register my phone. Thus, after the OS upgrade I was not able to use this option to re-register my PC again.
I then wrote to the Nokia Care Line stating the issue. They took about 5 days and the re-registering link was reset for me in my Ovi Music Account. I had to provide them with my IMEI number and Ovi Account Id. I sometimes wonder how Ovi Music really works. Shouldn't it track your PC or Phone by some hardware id to avoid such issues. Below is the email chain of the whole issue.
To find the Nokia Care Line page for your country, simply launch the website for your country, click on the Contact Us link. Now look for link which allows you to email Nokia. You will be presented with a form where you can mention your issue and provide all the details.