There may be some registry entries that shows default path for Opus wrong. Personally I don't like when programs install in two separate folders (I think it's useless in 99% cases and directory name doesn't determine type of execute file; I also like to see all my programs in one folder), so I created junction "progs" that point to "Program Files" directory, because some programs install in x86 folder anyway, no matter if you force them to install in main "Program Files". If you want to force programs to install in "Program Files", make junction (junction c:\progs "c:\Program Files" - technet.microsoft.com/pl-pl/sys ... 96768.aspx in case that your system doesn't native support this) and select c:\progs\ as installation directory.
If you have no Opus installed, check registry for "Program Files (x86)\GPSoftware" - maybe there is some old entry with that wrong path somewhere.
Another strange idea is to create new windows administrator account, switch users then try to install Opus (for all users).
All my advices are just guessing, unfortunatelly. I'm just want to help by give some ideas.