In the English language, the plural of virus is viruses.

In reference to a computer virus, the plural is often believed to be virii or, less commonly, viri, but both forms are neologistic folk etymology[1] and no major dictionary recognizes them as alternative forms. These variations, discussed below, are generally used by people in some Internet communities to refer to computer malware of any type, not just self replicating and self distributing forms.


source:http://en.wikipedia.org/wiki/Plural_of_virus