Start -> Run -> explorer.exe
(If Opus is in Explorer replacement mode then double-clicking folders in Explorer will end up opening Opus which isn't ideal if you're aiming to navigate to dopus.exe to run it in a special way. You can work around that by nagivating using Explorer's tree, or typing the path into the address bar, or right-clicking folders and selecting Open instead of double-clicking them.)
You could also make a batch file or shorcut, maybe for your desktop or start menu, which uses runas.exe to launch dopus.exe as another user, saving you the hassle of lots of clicking.
If you know VBScript then you could also make an Opus button which runs a script which tells Opus to exit, waits for Opus to finish exiting, and then runs dopus.exe using runas.exe (or using ShellExecute and the Runas verb).
I've got some VBScript that stops Opus and waits for it to finish exiting (using WMI to wait until the dopus.exe exits) if it's any use to you. It also runs something (not Opus) as administrator. (It's an Opus button I have which shuts down Opus, copies a new version over under an administrator account, then restarts Opus under the normal user account.) If you don't know VBScript then it won't be useful though.