You mean, you can't start programs from Opus? Or does only the »open with« fail? Do you maybe use some security suite. Much of them use »parent control« functions,
which could wrongly block Opus to start programs through it. That's the only idea i have about this.
The one at the bottom of the page is probably worth trying first, though it depends what is messed up. Seems a number of things can cause this.
Also, I'd recommend not using the .reg files they provide in case they're different for different Windows versions. Best to compare the the registry areas the guide points to against the working machine to see what's different and avoid importing any .reg files.
FWIW, Microsoft also have an article on this, although it's missing many of the suggestions on the one above: