Since you have upgraded your PC with a new operating system, it had been detedted as a new PC.
With the new system update, you can remove old inaccessible security information on your account automatically after 30 days. To do that:
- Log in to this link.
- Click Delete on the security information you do not have access to. (Alternate email, Trusted PC, Mobile number)
- Select "No, I Don't have access" button. (The date on when the security information will be removed will appear )
- Click Delete Security Info.
- Update the account with new Security Info.
- Click Save.
You just need to wait for 30 days for all the information to be removed and the new security information to be automatically confirmed.
Try this workaround in adding the new PC:
Hit Windows Icon on your computer +R.
Type wlstartup.exe and click Ok.
It will lauch Windows Live Essentials 2011.
Log in and select Make this my Trusted PC.