If the admin mode buttons are disabled it means Opus either thinks it is running on Windows XP (which can happen if it has been configured to run in XP compatibility mode) or the whole process is running as administrator (in which case you can't elevate; that seems unlikely as you cannot create folders in C:\Program Files).
Have you checked that Opus is not running elevated (as admin), and that dopus.exe is not set to run with any compatibility settings?
These two FAQs may help, in addition to the information in the thread you linked:
[ul][li]Why NOT to run Opus as Administrator in Vista / Windows 7
[/li]
[li]Installer says XP SP3 is required on a later OS version[/li][/ul]
It may also be worth a try to re-install Opus over the top of itself (don't uninstall first, unless you've made a backup of your configuration), in case the components used for UAC elevation don't have their correct registry settings.