When one installs a modern Opera version (like v33) on a computer, if there is an old (Presto) Opera version already present (v12.17 was the latest), the installer can bring in bookmarks from the old installation, but it will leave the old version in place and functional on the system. Opera 12 and older have their own internal updater which can independently notify a user that a new Opera version is available, either in the 12 series or the availability of an entirely new Opera version series, and offer to install it. It sounds as if a system-present version 12 or lower is making such an update notification. However, if true, that version 12 had to get onto the system somehow, and the most likely explanation is that it was already there when you installed (or updated to) the new Opera. In that case, you should either uninstall the old Opera version or disable its updater file, depending on whether you want to keep the old version. It's also possible that if you upgraded the OS to Win10, it set off some Opera updating triggers that were otherwise disabled or dormant under the prior OS.

In any case, a clean installation of a modern Opera version onto an Opera-free system should neither install version 12 nor set in motion an update notification to attempt a v12 install. In such a case, something definitely abnormal is occurring, either functionally or in the malware realm.