Is there an 'anti-virus'?
There is a &quot;anti-virus,&quot; although many call it a virophage.In 2008, a paper in Nature was published about the observations of a new strain of a virus known as Acanthamoeba polyphaga mimivirus. This virus mainly attacks amoeba. It was discovered in 1992. It was one of the biggest viruses ever found.Later, a related virus called the mamavirus was discovered. But, after observing this specimen under an electron microscope, scientist found tiny viral particles attacking the mamavirus. It was called Sputnik. The Sputnik virus hijacks the mamvirus's machinery and depends on the mamavirus to survive. It made scientists wonder if the mamavirus is a living thing.
Here is a picture of it attacking the mamavirus:

You can see small subviral particles attacking the bigger virus. That is Sputnik.
Note that it is a virus that coinfects other organisms but needs a virus infecting an organism to survive. According to Wikipedia:

Sputnik virophage is a subviral agent that reproduces in amoeba cells that are already infected by a certain helper virus; Sputnik uses the helper virus's machinery for reproduction and inhibits replication of the helper virus.

It seems that there are more species of virophages, including one that infects the marine phagotropic flagellate Cafeteria roenbergensis in the presence of a second virus — Cafeteria roenbergensis virus and another one known as the Organic Lake virophage but not much detail is known about this virus.
Here is the link to the Nature article on the Sputnik virophage, published in 2008:
http://www.nature.com/nature/journal/v455/n7209/full/nature07218.html
It may be possible to synthesize one in the future. However using this to kill viruses would not make sense because the virophage technically still attacks the host of the virus it is hijacking, plus, from what I understand, it only uses the virus's machinery.
Hopefully this was helpful. If you have questions, you can comment on this answer...