Symantec, the link you posted, seems to classify this as a virus.

Why (or why not) is it?

Yes, although "virus" is used in very general terms to mean "porgrams that secretly do bad things to your computer" (more accurately called "malware"), it has a more precise definition, and Opener does not fit it.

Roughly:

*Virus - A program that, like a biological virus, automatically replicates itself.

*Worm - Very similar to a virus, but it actively tries to infect other computers (for example by emailing itself to other people or via network backdoors). Almost all the big Windows nasties are, therefore, technically worms.

*Trojan - A program that pretends to do one thing, but actually does something else (usually installs some other malware or virus).

Coming back to Opener: as explained above, it needs to be installed by someone (or something) with administrator-level access to your computer, so by itself it is none of these things.

It does not (so far as I have read) replicate itself, and it certainly does not have any means of spreading itself beyond the computer on which it is installed and any connected machines (if it can even do that, in which case it would, I suppose, be a virus, albeit a very limited one since it is only installed in specific locations).

Symantec is clearly wrong about classifying Opener as a virus then?