Update:
Fixed in Opus 9.1.3.1
Old/original reply:
Right now (Opus 9.1.3.0) Explorer Replacement doesn't work properly in Windows 7 so the options to turn it on and off are hidden.
Unfortunately, if you import a configuration from another machine where Explorer Replacement is on, you'll end up with it on in Windows 7 as well, and still have no way to turn it off in Preferences.
GPSoftware are working on a free update which will make Explorer Replacement work properly on Windows 7 and restore the preferences options.
Until then, if you're confident using RegEdit, you can manually disable Explorer Replacement by editing the registry:
[ul][li]How to manually remove Explorer Replacement settings.[/li][/ul]
If you're not confident using RegEdit, shout. I could probably create a script which makes the changes for you.